Chart theming: user font upload (FontFace-from-IndexedDB) + variable-font weight support

docs/IMPLEMENTATION-PLAN.md

@@ -514,9 +514,13 @@ is in the maintained plan, not the archive.
 
 **Chart theming** (`exploration/chart-theming-scope.md`):
 
-- **User font upload** — FontFace-from-IndexedDB tier; theme `fonts: { family, source: 'file' }`.
+- ~~**User font upload** — FontFace-from-IndexedDB tier~~ ✅ (incl. variable-font weight
+  support; arch 05 → User-uploaded fonts). Font dependencies derive from the config, not a
+  theme field.
 - **Google Fonts opt-in CDN tier** — keyless catalog, opt-in only.
-- **SVG export font embedding** — embed face data so exported SVGs render off-app.
+- **SVG export font embedding** — embed face data so exported SVGs render off-app; carries
+  the **§08 export font-byte round-trip** too (shared base64 machinery — user fonts don't yet
+  travel with an exported workspace).
 - **Theme↔font pairing metadata** — a suggestion nicety.
 - **Built-in expressive theme preset gallery** — e.g. "Editorial", "Terminal", "Sketch".
 

docs/architecture/05-rendering-theming-preview.md

@@ -149,6 +149,8 @@ async function rerender(node: HTMLElement, spec: TopLevelSpec, config: Config) {
   swallow under §7's fail-loud rule). Chart fonts are self-hosted in
   `styles/chart-fonts.css` (offered by the Theme Builder's font control); only
   their latin subsets are precached, the rest runtime-cached (vite.config Workbox).
+  User-uploaded faces are registered on `document.fonts` too, so the same gate
+  resolves them (§3 → User-uploaded fonts).
 - **Do** call `view.finalize()` on every previous view before rendering a new
   one, and on component unmount.
 - **Do** keep exactly one live view per preview node.
@@ -248,6 +250,41 @@ key-by-key), so a snippet can always override or opt out locally. The
 that boundary deliberately: merge bakes the selected theme into `spec.config`
 (spec keys win — rendering unchanged), extract lifts `spec.config` out.
 
+### User-uploaded fonts
+
+Beyond the self-hosted roster, a user can upload font files (`.woff2`/`.woff`/
+`.ttf`/`.otf`) for chart themes. A `FontAsset` (`core/font-asset.ts`) holds the
+raw bytes and persists through the standard entity-store stack — a `fonts`
+IndexedDB store, `infrastructure/font-store.ts` (+ `font-migrations.ts`),
+`stores/FontStore.ts`, `orchestration/font-persistence.ts` — the same
+one-tier-each shape as datasets and custom themes. The browser seam that turns
+stored bytes into a live face, `infrastructure/font-faces.ts`, registers on
+`document.fonts` at startup (before the first render, so the §2 font gate
+resolves user faces like the roster) and on each upload; uploads/deletes are
+orchestrated by `services/fonts.ts`. The Type panel lists user fonts ahead of the
+roster and applies one via the same `applyFontToConfig` transform.
+
+**Font dependencies are derived from the config, never stored on the theme.** A
+`CustomTheme` carries no font field: the family a theme (or a snippet) uses is
+already in its config's `font`/`*Font` slots, that JSON config is the source of
+truth, and snippets use uploaded fonts with no theme to carry such a field. So a
+used face is discovered by scanning configs/specs for the families they reference
+(`collectFontFamilies`, core) and matching against the font library — a stored
+field would only drift from the config it duplicates.
+(Embedding the matched faces' bytes into the §08 workspace export and per-chart
+SVG export is the remaining transfer work, on shared base64 machinery.)
+
+**Variable fonts: weight is the only leverageable axis.** `parseFontAxes` reads
+the OpenType `fvar` table (uncompressed `ttf`/`otf` only — `woff`/`woff2` wrap
+their tables in compression we don't unpack, so those register as static), and a
+variable face is registered with its `wght`→`weight` and `wdth`→`stretch` ranges
+so one file serves the whole weight range, driven by the Type panel's weight
+controls. Only those two axes take effect because Vega's text rendering emits a
+CSS font shorthand (family/size/weight/style) with **no `font-variation-settings`
+hook** — optical size, grade, and custom axes pin at the registered default and
+can't be exposed. Declaring the `wdth` range also defaults the face to normal
+width rather than a variable font's possibly-condensed default instance.
+
 ### Structured controls
 
 The builder's panels — Color, Type, Layout, Axes & grid, Legend

docs/exploration/chart-theming-scope.md

@@ -162,10 +162,20 @@ ships, it is an explicit per-font user action, never automatic.
    subset of the UI Plex Sans/Mono, and runtime-caches the rest (latin-ext + non-latin)
    CacheFirst so a script works offline after first use. Roster picked from a visual
    specimen. Not done here: theme↔font pairing metadata (a suggestion nicety, deferred).
-6. **User font upload** — FontFace-from-IndexedDB tier; theme entity's `fonts` field
-   carries `{ family, source: 'file' }`.
-7. **Deferred** — Google Fonts opt-in tier; SVG export font embedding; built-in
-   expressive preset gallery ("Editorial", "Terminal", "Sketch") showcasing the roster.
+6. **User font upload** ✅ (2026-06-16) — `FontAsset` (`core/font-asset.ts`) + the `fonts`
+   store @ DB v3, registered as a `FontFace` at startup so the render gate resolves user
+   faces like the roster (full entity-store stack: adapter/migration, `FontStore`,
+   `font-persistence`, `services/fonts`; arch 05 → User-uploaded fonts). The Type panel
+   offers uploads ahead of the roster. **Variable fonts** are supported: `parseFontAxes`
+   reads `fvar` (uncompressed ttf/otf) and the face registers with `wght`/`wdth` ranges, so
+   one file drives the whole weight range — only weight/width survive Vega's text rendering
+   (no `font-variation-settings` hook). No `CustomTheme.fonts` field: a used face is derived
+   by scanning configs/specs (`collectFontFamilies`) — the config is the
+   source of truth, and snippets use fonts with no theme to carry a field. Not done here:
+   embedding font bytes into the §08 export + SVG export (shared base64 machinery, with item 7).
+7. **Deferred** — Google Fonts opt-in tier; SVG export font embedding (+ §08 font-byte
+   round-trip, shared machinery); built-in expressive preset gallery ("Editorial",
+   "Terminal", "Sketch") showcasing the roster.
 
 **Rejected:** per-snippet theme field (2026-06-12 — `spec.config` + merge/extract covers
 it without a second mechanism).
@@ -224,6 +234,14 @@ the app standardizes on v6.
 
 ## 6. Status log
 
+- **2026-06-16 (slice 6)** — **user font upload + variable-font weight support.** New
+  `FontAsset` entity (instance #4 of the entity-store kind, confirmed by an eng-council
+  pre-build consult): `fonts` store @ DB v3, adapter/migration, `FontStore`,
+  `font-persistence`, `services/fonts`, and the `font-faces` `FontFace`-registration seam
+  wired into `startup`. Type panel gains an upload control + a managed list; user fonts lead
+  the dropdown. Variable fonts parse `fvar` and register with `wght`/`wdth` ranges (weight
+  is the only axis Vega's text rendering can drive). Font dependencies are derived from the
+  config at export, not stored on the theme. Deferred: font bytes in the §08/SVG exports.
 - **2026-06-14 (Color panel bugfix)** — **scheme picks rendered blank.** A named scheme
   was written into `config.range.*` as a bare string, which vega-lite compiles but Vega
   rejects at render ("Unrecognized scale range value") — silently caught by the gallery's

src/app/components/ThemeFields.module.css

@@ -75,3 +75,53 @@
   font-size: 10px;
   color: var(--text-secondary);
 }
+
+/* Managed list of user-uploaded fonts under the Type panel's font control. */
+.fontList {
+  display: grid;
+  gap: var(--space-2);
+  margin: var(--space-2) 0 0;
+  padding: 0;
+  list-style: none;
+}
+
+.fontItem {
+  display: grid;
+  grid-template-columns: 1fr auto auto;
+  align-items: center;
+  gap: var(--space-3);
+}
+
+.fontNameWrap {
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
+  min-width: 0;
+}
+
+.fontName {
+  font-size: 13px;
+  color: var(--text);
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+/* "Variable" tag on a variable-font row in the managed list — passive chrome,
+   bordered rather than filled (arch 09 §4). */
+.fontBadge {
+  flex: none;
+  padding: 1px var(--space-2);
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  color: var(--text-secondary);
+  font-size: 11px;
+  font-variant-numeric: tabular-nums;
+  white-space: nowrap;
+}
+
+.fontMeta {
+  font-size: 12px;
+  color: var(--text-secondary);
+  font-variant-numeric: tabular-nums;
+}

src/app/components/TypeControls.tsx

@@ -8,10 +8,17 @@
  * the per-domain panels (Axes, Legend); this panel is family, size, and weight.
  */
 
+import { useMemo, useRef } from 'react';
 import { THEME_FONT_OPTIONS } from '@core/custom-theme';
+import { type FontAxis, fontFamilyStack, isVariableFont } from '@core/font-asset';
 import type { JsonObject } from '@core/spec-config';
+import { humanizeBytes } from '@core/storage-estimate';
 import { asNumber, type ConfigPath, getConfigValue } from '@core/theme-controls';
+import { removeFont, uploadFontFiles } from '../services/fonts';
+import { confirm } from '../stores/ConfirmStore';
 import { useConfigSetter, useCustomThemeStore } from '../stores/CustomThemeStore';
+import { useFontStore } from '../stores/FontStore';
+import { Button } from './Button';
 import { SelectControl, type SelectControlOption } from './SelectControl';
 import { ControlSection, NumberRow, SelectRow } from './ThemeFields';
 import styles from './ThemeFields.module.css';
@@ -24,24 +31,36 @@ const AXIS_LABEL_SIZE: ConfigPath = ['axis', 'labelFontSize'];
 const AXIS_LABEL_WEIGHT: ConfigPath = ['axis', 'labelFontWeight'];
 
 /**
- * Font options, each labelled in its own family so the dropdown previews the
- * typeface (the type analogue of the color dropdowns' swatches). The roster
- * (THEME_FONT_OPTIONS) is loaded before render by the chart-renderer's font gate.
+ * The built-in roster, each labelled in its own family so the dropdown previews
+ * the typeface (the type analogue of the color dropdowns' swatches). Loaded
+ * before render by the chart-renderer's font gate. User-uploaded fonts are merged
+ * ahead of these at render (see `TypeControls`).
  */
-const FONT_OPTIONS: SelectControlOption<string>[] = THEME_FONT_OPTIONS.map(({ value, label }) => ({
-  value,
-  label,
-  labelStyle: { fontFamily: value },
-}));
+const ROSTER_FONT_OPTIONS: SelectControlOption<string>[] = THEME_FONT_OPTIONS.map(
+  ({ value, label }) => ({ value, label, labelStyle: { fontFamily: value } }),
+);
 
-// Numeric font weights as a tri-state enum; '' is the unset (default) sentinel.
-type Weight = '' | '400' | '500' | '600' | '700' | 'custom';
+/** A variable font's badge text — its weight range when present, else just "Variable". */
+function variableBadge(axes: FontAxis[] | undefined): string {
+  const wght = axes?.find((a) => a.tag === 'wght');
+  return wght ? `Variable ${wght.min}–${wght.max}` : 'Variable';
+}
+
+// Numeric font weights as an enum; '' is the unset (default) sentinel. The full
+// 100–900 range is offered so a variable font's weight axis is fully selectable;
+// a static face simply faux-renders the weights it doesn't physically carry.
+type Weight = '' | '100' | '200' | '300' | '400' | '500' | '600' | '700' | '800' | '900' | 'custom';
 const weightOptions: SelectControlOption<Weight>[] = [
   { value: '', label: 'Theme default' },
+  { value: '100', label: 'Thin' },
+  { value: '200', label: 'Extra Light' },
+  { value: '300', label: 'Light' },
   { value: '400', label: 'Normal' },
   { value: '500', label: 'Medium' },
   { value: '600', label: 'Semibold' },
   { value: '700', label: 'Bold' },
+  { value: '800', label: 'Extra Bold' },
+  { value: '900', label: 'Black' },
 ];
 const weightValue = (v: unknown): Weight => {
   if (v === undefined) return '';
@@ -57,22 +76,58 @@ const weightValue = (v: unknown): Weight => {
 
 export function TypeControls({ config }: { config: JsonObject }) {
   const set = useConfigSetter();
+  const fonts = useFontStore((s) => s.fonts);
+  const fileRef = useRef<HTMLInputElement>(null);
+
+  // User uploads first, then a divider, then the built-in roster — so a brand's
+  // own faces lead the list (SelectControl's `dividerBefore` draws the boundary).
+  const fontOptions = useMemo<SelectControlOption<string>[]>(() => {
+    const userOptions = fonts.map((f) => ({
+      value: fontFamilyStack(f.family),
+      label: f.family,
+      labelStyle: { fontFamily: fontFamilyStack(f.family) },
+    }));
+    const roster = ROSTER_FONT_OPTIONS.map((o, i) =>
+      i === 0 && userOptions.length > 0 ? { ...o, dividerBefore: true } : o,
+    );
+    return [...userOptions, ...roster];
+  }, [fonts]);
+
   const currentFont =
-    typeof config.font === 'string' ? FONT_OPTIONS.find((o) => o.value === config.font) : undefined;
+    typeof config.font === 'string' ? fontOptions.find((o) => o.value === config.font) : undefined;
 
   const setWeight = (path: ConfigPath) => (w: Weight) =>
     set(path, w === '' ? undefined : Number(w));
 
+  const onPickFiles = (e: React.ChangeEvent<HTMLInputElement>) => {
+    const picked = e.target.files;
+    if (picked && picked.length > 0) void uploadFontFiles(picked);
+    e.target.value = ''; // let the same file be re-picked after a removal
+  };
+
+  const onRemoveFont = async (id: number, family: string) => {
+    const ok = await confirm({
+      title: 'Remove font',
+      message:
+        `Remove "${family}"? Charts using it fall back to a default font. ` +
+        'This only removes the uploaded file from Astrolabe.',
+      confirmLabel: 'Remove',
+      danger: true,
+    });
+    if (ok) removeFont(id);
+  };
+
   return (
     <div className={styles.panel}>
       <ControlSection title="Font family" hint="Applied to every text slot in the config.">
         <div className={styles.field}>
           <span className={styles.fieldLabel}>Font</span>
+          <div className={styles.control}>
             <SelectControl
               id="theme-builder-font"
               label="Font family"
               heading="Apply font"
-            options={FONT_OPTIONS}
+              options={fontOptions}
               value={currentFont?.value}
               onSelect={(family) => useCustomThemeStore.getState().applyDraftFont(family)}
               triggerContent={
@@ -87,7 +142,46 @@ export function TypeControls({ config }: { config: JsonObject }) {
               }
               triggerTitle="Write one font family into every font slot of the config"
             />
+            <Button variant="ghost" onClick={() => fileRef.current?.click()}>
+              Upload font…
+            </Button>
+            <input
+              ref={fileRef}
+              type="file"
+              accept=".woff2,.woff,.ttf,.otf"
+              multiple
+              hidden
+              onChange={onPickFiles}
+            />
           </div>
+        </div>
+        {fonts.length > 0 && (
+          <ul className={styles.fontList}>
+            {fonts.map((f) => (
+              <li key={f.id} className={styles.fontItem}>
+                <span className={styles.fontNameWrap}>
+                  <span
+                    className={styles.fontName}
+                    style={{ fontFamily: fontFamilyStack(f.family) }}
+                  >
+                    {f.family}
+                  </span>
+                  {isVariableFont(f.axes) && (
+                    <span className={styles.fontBadge}>{variableBadge(f.axes)}</span>
+                  )}
+                </span>
+                <span className={styles.fontMeta}>{humanizeBytes(f.size)}</span>
+                <Button
+                  variant="ghost"
+                  onClick={() => void onRemoveFont(f.id, f.family)}
+                  aria-label={`Remove ${f.family}`}
+                >
+                  Remove
+                </Button>
+              </li>
+            ))}
+          </ul>
+        )}
       </ControlSection>
 
       <ControlSection title="Title">

src/app/infrastructure/db.test.ts

@@ -8,6 +8,7 @@ import { afterEach, beforeEach, describe, expect, it } from 'vitest';
 import { IDBFactory } from 'fake-indexeddb';
 import {
   DATASETS_STORE,
+  FONTS_STORE,
   SNIPPETS_STORE,
   THEMES_STORE,
   _resetDbForTests,
@@ -16,7 +17,7 @@ import {
   put,
 } from './db';
 
-const ALL_STORES = [SNIPPETS_STORE, DATASETS_STORE, THEMES_STORE];
+const ALL_STORES = [SNIPPETS_STORE, DATASETS_STORE, THEMES_STORE, FONTS_STORE];
 
 /** Open the raw database at `version` with a custom (or absent) upgrade body. */
 function rawOpen(version: number, upgrade?: (db: IDBDatabase) => void): Promise<IDBDatabase> {

src/app/infrastructure/db.ts

@@ -11,16 +11,18 @@ const DB_NAME = 'astrolabe';
 /**
  * Store-layout version. Bump only when the set of object stores / indexes
  * changes — independent of per-record schema versions (see snippet-migrations).
- * v2 added the `themes` store (custom chart themes).
+ * v2 added the `themes` store (custom chart themes); v3 added the `fonts` store
+ * (user-uploaded font faces — binary bytes, stored verbatim by structured clone).
  */
-const DB_VERSION = 2;
+const DB_VERSION = 3;
 
 export const SNIPPETS_STORE = 'snippets';
 export const DATASETS_STORE = 'datasets';
 export const THEMES_STORE = 'themes';
+export const FONTS_STORE = 'fonts';
 
 /** Every object store the app expects — the open-time verification checklist. */
-const EXPECTED_STORES = [SNIPPETS_STORE, DATASETS_STORE, THEMES_STORE] as const;
+const EXPECTED_STORES = [SNIPPETS_STORE, DATASETS_STORE, THEMES_STORE, FONTS_STORE] as const;
 
 let dbPromise: Promise<IDBDatabase> | null = null;
 

src/app/infrastructure/file-transfer.ts

@@ -35,6 +35,11 @@ export function readTextFile(file: File): Promise<string> {
   return file.text();
 }
 
+/** Read a picked file's raw bytes (rejects on an unreadable file) — for font uploads. */
+export function readBinaryFile(file: File): Promise<ArrayBuffer> {
+  return file.arrayBuffer();
+}
+
 /** Copy `text` to the clipboard (rejects when the browser blocks access). The
  *  one place outside a component that touches the clipboard API. */
 export function copyText(text: string): Promise<void> {

src/app/infrastructure/font-faces.ts

@@ -0,0 +1,57 @@
+/**
+ * FontFace registration seam (docs/architecture/02; arch 00 — only
+ * infrastructure touches browser APIs).
+ *
+ * Turns stored FontAsset bytes into live `FontFace`s on `document.fonts`, so the
+ * chart renderer's font gate (`ensureFontsLoaded` in chart-renderer.ts) and the
+ * Type-panel dropdown resolve user fonts exactly like the @fontsource roster.
+ * Registration is keyed by font id (so a delete removes precisely the right
+ * face), idempotent (re-registering an id replaces its face), and a no-op where
+ * the Font Loading API is absent (tests, SSR).
+ *
+ * A static file is one weight, registered with default descriptors (normal/400);
+ * a config requesting bold falls back to the browser's faux-bold. A **variable**
+ * file is registered with its parsed weight/width ranges (`variableFontDescriptors`),
+ * so the whole weight axis renders from one upload and the Type panel's weight
+ * controls drive it for real.
+ */
+
+import { type FontAsset, variableFontDescriptors } from '@core/font-asset';
+
+/** id → the live face we added, so we can remove exactly it on delete/replace. */
+const registered = new Map<number, FontFace>();
+
+/** Register (or replace) a stored font as a live `FontFace` on `document.fonts`. */
+export function registerFontAsset(asset: FontAsset): void {
+  if (typeof document === 'undefined' || !document.fonts) return;
+  unregisterFont(asset.id); // replace any prior face for this id (re-import)
+  let face: FontFace;
+  try {
+    // Variable fonts get weight/width ranges so the browser resolves a requested
+    // weight against the axis; a static font passes no descriptors (normal/400).
+    face = new FontFace(asset.family, asset.data, variableFontDescriptors(asset.axes));
+  } catch {
+    return; // a malformed family name or unreadable source — nothing to register
+  }
+  registered.set(asset.id, face);
+  document.fonts.add(face);
+  // Decode eagerly so the next chart render finds the face ready. A decode
+  // failure is non-fatal — the chart renders with fallback metrics — which is
+  // arch 02's sanctioned graceful-fallback case, kept explicit, not buried.
+  face.load().catch(() => {});
+}
+
+/** Register a batch (startup hydration). */
+export function registerFontAssets(assets: ReadonlyArray<FontAsset>): void {
+  for (const asset of assets) registerFontAsset(asset);
+}
+
+/** Remove a previously-registered face by font id. No-op if not registered. */
+export function unregisterFont(id: number): void {
+  if (typeof document === 'undefined' || !document.fonts) return;
+  const face = registered.get(id);
+  if (face) {
+    document.fonts.delete(face);
+    registered.delete(id);
+  }
+}

src/app/infrastructure/font-migrations.test.ts

@@ -0,0 +1,68 @@
+import { describe, expect, it } from 'vitest';
+import { CURRENT_FONT_VERSION } from '@core/font-asset';
+import { migrateFontAsset } from './font-migrations';
+
+const buf = (n: number): ArrayBuffer => new Uint8Array(n).buffer;
+
+describe('migrateFontAsset', () => {
+  it('passes a current record through unchanged (plus version stamp)', () => {
+    const record = {
+      id: 4,
+      version: CURRENT_FONT_VERSION,
+      family: 'Brand Sans',
+      data: buf(2048),
+      format: 'woff2' as const,
+      fileName: 'Brand-Sans.woff2',
+      source: 'file' as const,
+      size: 2048,
+      created: '2026-06-15T10:00:00.000Z',
+      modified: '2026-06-15T10:00:00.000Z',
+    };
+    expect(migrateFontAsset(record)).toEqual(record);
+  });
+
+  it('fills missing or invalid fields with safe defaults', () => {
+    const migrated = migrateFontAsset({ id: '9', fileName: 'x.ttf' });
+    expect(migrated.id).toBe(9);
+    expect(migrated.version).toBe(CURRENT_FONT_VERSION);
+    expect(migrated.family).toBe('Custom font');
+    expect(migrated.format).toBe('ttf'); // recovered from the file extension
+    expect(migrated.data).toBeInstanceOf(ArrayBuffer);
+    expect(migrated.size).toBe(0);
+    expect(migrated.source).toBe('file');
+  });
+
+  it('keeps valid variation axes and drops a malformed axes value', () => {
+    const withAxes = migrateFontAsset({
+      id: 2,
+      family: 'Fixel',
+      data: buf(1),
+      format: 'ttf',
+      fileName: 'Fixel.ttf',
+      axes: [{ tag: 'wght', min: 100, default: 400, max: 900 }],
+    });
+    expect(withAxes.axes).toEqual([{ tag: 'wght', min: 100, default: 400, max: 900 }]);
+
+    const badAxes = migrateFontAsset({
+      id: 3,
+      family: 'X',
+      data: buf(1),
+      format: 'ttf',
+      fileName: 'x.ttf',
+      axes: 'not-an-array',
+    });
+    expect(badAxes.axes).toBeUndefined();
+  });
+
+  it('keeps unknown fields written by a newer build', () => {
+    const migrated = migrateFontAsset({
+      id: 1,
+      family: 'Next',
+      data: buf(1),
+      format: 'otf',
+      fileName: 'n.otf',
+      futureField: 'kept',
+    });
+    expect((migrated as unknown as Record<string, unknown>).futureField).toBe('kept');
+  });
+});

src/app/infrastructure/font-migrations.ts

@@ -0,0 +1,59 @@
+/**
+ * Read-time migration for FontAsset records (docs/architecture/02 §4).
+ *
+ * Mirrors snippet/dataset/theme migrations: every font read from storage passes
+ * through `migrateFontAsset`, which fills missing/invalid fields and stamps the
+ * current version. Unknown fields are tolerated (spread the original, only fill
+ * gaps) so a record written by a newer build round-trips without loss.
+ */
+
+import {
+  CURRENT_FONT_VERSION,
+  detectFontFormat,
+  type FontAsset,
+  type FontAxis,
+  type FontFormat,
+  type FontSource,
+} from '@core/font-asset';
+
+const FORMATS: ReadonlyArray<FontFormat> = ['woff2', 'woff', 'ttf', 'otf'];
+
+/** Accept a stored `axes` array only if it's shaped like FontAxis records. */
+function migrateAxes(raw: unknown): FontAxis[] | undefined {
+  if (!Array.isArray(raw)) return undefined;
+  const axes = raw.filter(
+    (a): a is FontAxis =>
+      !!a &&
+      typeof (a as FontAxis).tag === 'string' &&
+      typeof (a as FontAxis).min === 'number' &&
+      typeof (a as FontAxis).max === 'number',
+  );
+  return axes.length > 0 ? axes : undefined;
+}
+
+/** Upgrade a raw stored record to the current FontAsset shape. */
+export function migrateFontAsset(raw: unknown): FontAsset {
+  const r = { ...(raw as Record<string, unknown>) };
+  const fileName = typeof r.fileName === 'string' ? r.fileName : '';
+  const format: FontFormat = FORMATS.includes(r.format as FontFormat)
+    ? (r.format as FontFormat)
+    : (detectFontFormat(fileName) ?? 'woff2');
+  const data = r.data instanceof ArrayBuffer ? r.data : new ArrayBuffer(0);
+  const source: FontSource = r.source === 'google' ? 'google' : 'file';
+  const axes = migrateAxes(r.axes);
+  delete r.axes; // re-attach the validated value below (or omit it entirely)
+  return {
+    ...r,
+    id: typeof r.id === 'number' ? r.id : Number(r.id),
+    version: CURRENT_FONT_VERSION,
+    family: typeof r.family === 'string' ? r.family : 'Custom font',
+    data,
+    format,
+    fileName,
+    source,
+    ...(axes ? { axes } : {}),
+    size: typeof r.size === 'number' ? r.size : data.byteLength,
+    created: typeof r.created === 'string' ? r.created : new Date(0).toISOString(),
+    modified: typeof r.modified === 'string' ? r.modified : new Date(0).toISOString(),
+  };
+}

src/app/infrastructure/font-store.ts

@@ -0,0 +1,29 @@
+/**
+ * Font asset persistence adapter (docs/architecture/02; scope doc §4 → fonts).
+ *
+ * The typed seam between the FontStore and IndexedDB's `fonts` object store.
+ * Exposes plain async functions returning domain `FontAsset` objects and migrates
+ * every record on read — same contract as dataset-store / theme-store. The
+ * payload is binary (`ArrayBuffer`), stored verbatim by the structured-clone
+ * algorithm the generic `put`/`getAll` already use; no special handling needed.
+ */
+
+import { CURRENT_FONT_VERSION, type FontAsset } from '@core/font-asset';
+import { FONTS_STORE, del, getAll, put } from './db';
+import { migrateFontAsset } from './font-migrations';
+
+/** Load every uploaded font, upgrading each record to the current shape. */
+export async function loadFonts(): Promise<FontAsset[]> {
+  const records = await getAll<unknown>(FONTS_STORE);
+  return records.map(migrateFontAsset);
+}
+
+/** Persist a font asset at the current schema version. Propagates failures. */
+export async function saveFont(font: FontAsset): Promise<void> {
+  await put(FONTS_STORE, { ...font, version: CURRENT_FONT_VERSION });
+}
+
+/** Permanently remove a font asset by id. */
+export async function deleteFont(id: number): Promise<void> {
+  await del(FONTS_STORE, id);
+}

src/app/orchestration/font-persistence.ts

@@ -0,0 +1,47 @@
+/**
+ * User font persistence wiring (docs/architecture/01 §5; scope doc §4 → fonts).
+ *
+ * The font sibling of `theme-persistence.ts`: a startup subscriber that diffs the
+ * `fonts` array against the previous snapshot and writes upserts/deletes through
+ * to the IndexedDB adapter. The store stays browser-free; failures surface as a
+ * toast rather than silent loss. Fonts change on explicit add/delete, so there is
+ * no debounce.
+ */
+
+import { deleteFont, saveFont } from '../infrastructure/font-store';
+import { notify } from '../stores/NotificationStore';
+import { useFontStore } from '../stores/FontStore';
+
+type Unsubscribe = () => void;
+
+function fontError(op: 'save' | 'delete', err: unknown) {
+  notify({
+    kind: 'error',
+    title: op === 'delete' ? "Couldn't delete the font" : "Couldn't save the font",
+    message:
+      'A storage error stopped Astrolabe from completing the last font change, so it may not ' +
+      'survive a reload. If this keeps happening, your browser may be blocking local storage.',
+    detail: err instanceof Error ? `Font ${op} failed: ${err.name}: ${err.message}` : String(err),
+  });
+}
+
+/** Persist font upserts and deletions whenever the array changes. */
+export function wireFontPersistence(): Unsubscribe {
+  let prevFonts = useFontStore.getState().fonts;
+  return useFontStore.subscribe((s) => {
+    const next = s.fonts;
+    if (next === prevFonts) return;
+    const prev = prevFonts;
+    prevFonts = next;
+
+    for (const old of prev) {
+      if (!next.some((n) => n.id === old.id)) {
+        deleteFont(old.id).catch((err) => fontError('delete', err));
+      }
+    }
+    for (const n of next) {
+      const old = prev.find((p) => p.id === n.id);
+      if (old !== n) saveFont(n).catch((err) => fontError('save', err));
+    }
+  });
+}

src/app/orchestration/startup.ts

@@ -11,17 +11,22 @@
 import type { Snippet } from '@core/snippet';
 import type { Dataset } from '@core/dataset';
 import type { CustomTheme } from '@core/custom-theme';
+import type { FontAsset } from '@core/font-asset';
 import { loadSnippets } from '../infrastructure/snippet-store';
 import { loadDatasets } from '../infrastructure/dataset-store';
 import { loadCustomThemes } from '../infrastructure/theme-store';
+import { loadFonts } from '../infrastructure/font-store';
+import { registerFontAssets } from '../infrastructure/font-faces';
 import { storageErrorNotification } from '../services/storage-errors';
 import { notify } from '../stores/NotificationStore';
 import { useSnippetStore } from '../stores/SnippetStore';
 import { useDatasetStore } from '../stores/DatasetStore';
 import { useCustomThemeStore } from '../stores/CustomThemeStore';
+import { useFontStore } from '../stores/FontStore';
 import { wireSnippetPersistence } from './snippet-persistence';
 import { wireDatasetPersistence } from './dataset-persistence';
 import { wireThemePersistence } from './theme-persistence';
+import { wireFontPersistence } from './font-persistence';
 import { startRouting } from '../modals/UrlStateSync';
 import { startEventRouter } from './EventRouter';
 
@@ -61,15 +66,31 @@ export async function initApp(): Promise<void> {
     notify(storageErrorNotification('load', err));
   }
 
+  // User-uploaded font faces (scope doc §4). Same failure posture: a font that
+  // can't be loaded just won't be offered or rendered, falling back to the stack's
+  // generic family.
+  let fonts: FontAsset[] = [];
+  try {
+    fonts = await loadFonts();
+  } catch (err) {
+    notify(storageErrorNotification('load', err));
+  }
+
   useSnippetStore.getState().hydrate(snippets);
   useDatasetStore.getState().hydrate(datasets);
   useCustomThemeStore.getState().hydrate(themes);
+  useFontStore.getState().hydrate(fonts);
+
+  // Register the loaded faces on document.fonts BEFORE the first chart render so
+  // the renderer's font gate (ensureFontsLoaded) can resolve them like the roster.
+  registerFontAssets(fonts);
 
   // Wire persistence AFTER hydrate so write-through's baseline is the loaded set
   // — otherwise it would redundantly re-save every record on each startup.
   wireSnippetPersistence();
   wireDatasetPersistence();
   wireThemePersistence();
+  wireFontPersistence();
 
   // Routing starts AFTER hydrate so the on-load hash restore can resolve snippet
   // / dataset ids against the loaded stores (spec §01E, docs/architecture/04).

src/app/services/fonts.ts

@@ -0,0 +1,117 @@
+/**
+ * Font upload service (scope doc §4 → User font upload) — orchestration over the
+ * pure core helpers, the FontStore, and the browser seams.
+ *
+ * The deterministic work (format detection, family derivation, uniqueness) lives
+ * in `@core/font-asset` + `@core/naming`; this layer reads the file bytes through
+ * the infrastructure adapter, validates, commits to the store (the persistence
+ * subscriber writes through to IndexedDB), registers the live `FontFace`, and
+ * reports one summary toast. Deleting a font unregisters its face and removes the
+ * record (the subscriber deletes it from IndexedDB).
+ */
+
+import {
+  createFontAsset,
+  detectFontFormat,
+  familyNameFromFileName,
+  MAX_FONT_BYTES,
+  parseFontAxes,
+} from '@core/font-asset';
+import { makeUniqueName } from '@core/naming';
+import { humanizeBytes } from '@core/storage-estimate';
+import { readBinaryFile } from '../infrastructure/file-transfer';
+import { registerFontAsset, unregisterFont } from '../infrastructure/font-faces';
+import { notify } from '../stores/NotificationStore';
+import { useFontStore } from '../stores/FontStore';
+
+/** Why a picked file was skipped, for the summary toast. */
+interface Skip {
+  fileName: string;
+  reason: string;
+}
+
+/**
+ * Add one or more picked font files to the library. Each file is validated
+ * independently (unsupported format / over the size cap are skipped, not fatal),
+ * given a unique family name, stored, and registered. Reports how many were added
+ * and why any were skipped — never throws on a bad file.
+ */
+export async function uploadFontFiles(files: FileList | File[]): Promise<void> {
+  const list = Array.from(files);
+  if (list.length === 0) return;
+
+  const added: string[] = [];
+  const skipped: Skip[] = [];
+
+  for (const file of list) {
+    const format = detectFontFormat(file.name);
+    if (!format) {
+      skipped.push({ fileName: file.name, reason: 'unsupported format' });
+      continue;
+    }
+
+    let data: ArrayBuffer;
+    try {
+      data = await readBinaryFile(file);
+    } catch {
+      skipped.push({ fileName: file.name, reason: 'could not be read' });
+      continue;
+    }
+
+    if (data.byteLength > MAX_FONT_BYTES) {
+      skipped.push({
+        fileName: file.name,
+        reason: `larger than ${humanizeBytes(MAX_FONT_BYTES)}`,
+      });
+      continue;
+    }
+
+    // Unique within the library (and within this batch — the store grew on the
+    // previous iteration, so re-read its families each time).
+    const family = makeUniqueName(
+      familyNameFromFileName(file.name),
+      useFontStore.getState().fonts.map((f) => f.family),
+    );
+    // Detect variation axes (variable fonts) so the face registers with the
+    // right weight/width ranges — uncompressed ttf/otf only; others stay static.
+    const axes = parseFontAxes(data, format);
+    const stored = useFontStore
+      .getState()
+      .add(createFontAsset({ family, data, format, fileName: file.name, axes }));
+    registerFontAsset(stored);
+    added.push(family);
+  }
+
+  reportUpload(added, skipped);
+}
+
+/** Remove a font: unregister its live face, then drop the record. */
+export function removeFont(id: number): void {
+  unregisterFont(id);
+  useFontStore.getState().remove(id);
+}
+
+/** One summary toast covering what was added and what was skipped. */
+function reportUpload(added: string[], skipped: Skip[]): void {
+  const skips = skipped.length
+    ? `Skipped ${skipped.map((s) => `${s.fileName} (${s.reason})`).join(', ')}.`
+    : '';
+
+  if (added.length === 0) {
+    notify({
+      kind: skipped.length ? 'error' : 'info',
+      title: skipped.length ? "Couldn't add the font" : 'No fonts added',
+      message:
+        skips || 'Choose a .woff2, .woff, .ttf, or .otf file to add a font to your chart themes.',
+    });
+    return;
+  }
+
+  const noun = added.length === 1 ? 'font' : 'fonts';
+  const message = `Added ${added.join(', ')}. ${skips}`.trim();
+  notify({
+    kind: skipped.length ? 'warning' : 'success',
+    title: `Added ${added.length} ${noun}`,
+    message,
+  });
+}

src/app/services/transfer.ts

@@ -60,6 +60,10 @@ export function exportWorkspace(now: Date = new Date()): void {
     return;
   }
 
+  // TODO(fonts §08): the envelope does not yet carry user-uploaded FontAsset
+  // bytes, so a theme/snippet referencing an uploaded font imports on another
+  // machine with the fallback family. Embed the referenced faces (base64) here —
+  // best done with task #3's shared font-bytes→embeddable-string helper.
   const envelope = buildExportEnvelope(snippets, datasets, themes, { now });
   downloadJson(exportFilename(now), JSON.stringify(envelope, null, 2));
 

src/app/stores/FontStore.test.ts

@@ -0,0 +1,42 @@
+import { beforeEach, describe, expect, it } from 'vitest';
+import { createFontAsset, type FontAsset } from '@core/font-asset';
+import { useFontStore } from './FontStore';
+
+const asset = (family: string, id = 1): FontAsset =>
+  createFontAsset({
+    family,
+    data: new Uint8Array(8).buffer,
+    format: 'woff2',
+    fileName: `${family}.woff2`,
+    id,
+  });
+
+beforeEach(() => useFontStore.getState().reset());
+
+describe('FontStore', () => {
+  it('add assigns the next free id (one past the max)', () => {
+    const a = useFontStore.getState().add(asset('A'));
+    const b = useFontStore.getState().add(asset('B'));
+    expect(a.id).toBe(1);
+    expect(b.id).toBe(2);
+    expect(useFontStore.getState().fonts.map((f) => f.family)).toEqual(['A', 'B']);
+  });
+
+  it('addFonts reassigns ids and appends without clobbering', () => {
+    useFontStore.getState().add(asset('Existing'));
+    useFontStore.getState().addFonts([asset('X', 1), asset('Y', 1)]);
+    const fonts = useFontStore.getState().fonts;
+    expect(fonts.map((f) => f.id)).toEqual([1, 2, 3]);
+    expect(fonts.map((f) => f.family)).toEqual(['Existing', 'X', 'Y']);
+  });
+
+  it('remove drops only the matching id; hydrate replaces wholesale', () => {
+    const a = useFontStore.getState().add(asset('A'));
+    useFontStore.getState().add(asset('B'));
+    useFontStore.getState().remove(a.id);
+    expect(useFontStore.getState().fonts.map((f) => f.family)).toEqual(['B']);
+    useFontStore.getState().hydrate([asset('Loaded', 5)]);
+    expect(useFontStore.getState().fonts).toHaveLength(1);
+    expect(useFontStore.getState().fonts[0].family).toBe('Loaded');
+  });
+});

src/app/stores/FontStore.ts

@@ -0,0 +1,61 @@
+/**
+ * User font library (scope doc §4 → User font upload; spec §04 → Chart theme).
+ *
+ * The durable collection of uploaded font faces, referenced by `family` from a
+ * config's font slots. Same two-layer shape as DatasetStore / CustomThemeStore:
+ * low-level mutators (`add`/`addFonts`/`remove`) are the single place the `fonts`
+ * array changes; a startup subscriber writes them through to IndexedDB.
+ *
+ * Persistence is NOT done here — `font-persistence.ts` observes this store and
+ * writes through to the adapter — and neither is `FontFace` registration (a
+ * browser side effect, owned by `services/fonts.ts` + `infrastructure/font-faces`),
+ * so the store stays browser-free.
+ */
+
+import { create } from 'zustand';
+import type { FontAsset } from '@core/font-asset';
+
+export interface FontState {
+  fonts: FontAsset[];
+
+  /** Replace the library from storage. */
+  hydrate: (fonts: FontAsset[]) => void;
+  /** Add a fully-formed asset, assigning the next free id. Returns the stored record. */
+  add: (font: FontAsset) => FontAsset;
+  /** Batch add (import path): reassign ids from the store's authority and append. */
+  addFonts: (incoming: FontAsset[]) => void;
+  /** Remove a font by id. */
+  remove: (id: number) => void;
+  /** Reset to initial state (tests). */
+  reset: () => void;
+}
+
+/** Next free numeric id — one past the max (same contract as `nextDatasetId`/`nextThemeId`). */
+function nextFontId(fonts: ReadonlyArray<FontAsset>): number {
+  return fonts.reduce((max, f) => Math.max(max, f.id), 0) + 1;
+}
+
+export const useFontStore = create<FontState>((set, get) => ({
+  fonts: [],
+
+  hydrate: (fonts) => set({ fonts }),
+
+  add: (font) => {
+    const withId = { ...font, id: nextFontId(get().fonts) };
+    set((s) => ({ fonts: [...s.fonts, withId] }));
+    return withId;
+  },
+
+  addFonts: (incoming) => {
+    if (incoming.length === 0) return;
+    set((s) => {
+      let nextId = nextFontId(s.fonts);
+      const withIds = incoming.map((f) => ({ ...f, id: nextId++ }));
+      return { fonts: [...s.fonts, ...withIds] };
+    });
+  },
+
+  remove: (id) => set((s) => ({ fonts: s.fonts.filter((f) => f.id !== id) })),
+
+  reset: () => set({ fonts: [] }),
+}));

src/core/font-asset.test.ts

@@ -0,0 +1,170 @@
+import { describe, expect, it } from 'vitest';
+import {
+  CURRENT_FONT_VERSION,
+  createFontAsset,
+  detectFontFormat,
+  familyNameFromFileName,
+  type FontAxis,
+  fontFamilyStack,
+  isVariableFont,
+  parseFontAxes,
+  variableFontDescriptors,
+} from './font-asset';
+
+const bytes = (n: number): ArrayBuffer => new Uint8Array(n).fill(1).buffer;
+
+/**
+ * Build a minimal SFNT with an `fvar` table carrying the given axes, so the
+ * parser is exercised against real byte layout without committing a font binary.
+ * Layout: 12-byte sfnt header, one 16-byte table record ('fvar'), then the fvar
+ * table (16-byte header + 20 bytes per axis). Mirrors the offsets parseFontAxes
+ * reads — verified against a real Fixel variable file during development.
+ */
+function buildVariableSfnt(axes: Array<[string, number, number, number]>): ArrayBuffer {
+  const axisSize = 20;
+  const fvarHeaderLen = 16;
+  const fvarLen = fvarHeaderLen + axes.length * axisSize;
+  const tableOffset = 12 + 16; // sfnt header + one table record
+  const buf = new ArrayBuffer(tableOffset + fvarLen);
+  const v = new DataView(buf);
+  v.setUint32(0, 0x00010000); // sfntVersion (TrueType)
+  v.setUint16(4, 1); // numTables
+  for (let i = 0; i < 4; i++) v.setUint8(12 + i, 'fvar'.charCodeAt(i));
+  v.setUint32(12 + 8, tableOffset); // table record: offset
+  v.setUint32(12 + 12, fvarLen); // table record: length
+  v.setUint16(tableOffset, 1); // fvar major version
+  v.setUint16(tableOffset + 4, fvarHeaderLen); // axesArrayOffset
+  v.setUint16(tableOffset + 8, axes.length); // axisCount
+  v.setUint16(tableOffset + 10, axisSize); // axisSize
+  const base = tableOffset + fvarHeaderLen;
+  axes.forEach(([tag, min, def, max], i) => {
+    const a = base + i * axisSize;
+    for (let j = 0; j < 4; j++) v.setUint8(a + j, tag.charCodeAt(j));
+    v.setInt32(a + 4, min * 65536);
+    v.setInt32(a + 8, def * 65536);
+    v.setInt32(a + 12, max * 65536);
+  });
+  return buf;
+}
+
+describe('detectFontFormat', () => {
+  it('maps supported extensions, case-insensitively', () => {
+    expect(detectFontFormat('Inter.woff2')).toBe('woff2');
+    expect(detectFontFormat('Inter.WOFF')).toBe('woff');
+    expect(detectFontFormat('Inter.ttf')).toBe('ttf');
+    expect(detectFontFormat('Inter.OTF')).toBe('otf');
+  });
+
+  it('reads the last extension and rejects unsupported / extensionless names', () => {
+    expect(detectFontFormat('My.Font.woff2')).toBe('woff2');
+    expect(detectFontFormat('Inter.eot')).toBeNull();
+    expect(detectFontFormat('Inter')).toBeNull();
+    expect(detectFontFormat('')).toBeNull();
+  });
+});
+
+describe('familyNameFromFileName', () => {
+  it('drops the extension and normalizes separators', () => {
+    expect(familyNameFromFileName('Inter-Regular.ttf')).toBe('Inter Regular');
+    expect(familyNameFromFileName('my_cool_font.woff2')).toBe('my cool font');
+    expect(familyNameFromFileName('Spaced  Out.otf')).toBe('Spaced Out');
+  });
+
+  it('falls back when the name reduces to nothing', () => {
+    expect(familyNameFromFileName('.woff2')).toBe('Custom font');
+  });
+});
+
+describe('createFontAsset', () => {
+  it('stamps version/timestamps/size and defaults source to file', () => {
+    const now = new Date('2026-06-15T12:00:00.000Z');
+    const asset = createFontAsset({
+      family: 'My Font',
+      data: bytes(2048),
+      format: 'woff2',
+      fileName: 'My-Font.woff2',
+      now,
+      id: 7,
+    });
+    expect(asset).toMatchObject({
+      id: 7,
+      version: CURRENT_FONT_VERSION,
+      family: 'My Font',
+      format: 'woff2',
+      fileName: 'My-Font.woff2',
+      source: 'file',
+      size: 2048,
+      created: now.toISOString(),
+      modified: now.toISOString(),
+    });
+  });
+
+  it('carries an explicit source', () => {
+    expect(
+      createFontAsset({
+        family: 'G',
+        data: bytes(1),
+        format: 'woff2',
+        fileName: 'g.woff2',
+        source: 'google',
+      }).source,
+    ).toBe('google');
+  });
+});
+
+describe('fontFamilyStack', () => {
+  it('quotes the family and appends a generic fallback', () => {
+    expect(fontFamilyStack('My Font')).toBe('"My Font", sans-serif');
+  });
+});
+
+describe('parseFontAxes', () => {
+  it('reads variation axes from an SFNT fvar table (ttf/otf)', () => {
+    const sfnt = buildVariableSfnt([
+      ['wght', 100, 400, 900],
+      ['wdth', 75, 100, 100],
+    ]);
+    expect(parseFontAxes(sfnt, 'ttf')).toEqual([
+      { tag: 'wght', min: 100, default: 400, max: 900 },
+      { tag: 'wdth', min: 75, default: 100, max: 100 },
+    ]);
+    expect(parseFontAxes(sfnt, 'otf')).toHaveLength(2);
+  });
+
+  it('returns [] for a static font (no fvar table)', () => {
+    // A valid sfnt header claiming zero tables — no fvar to find.
+    const buf = new ArrayBuffer(12);
+    new DataView(buf).setUint32(0, 0x00010000);
+    expect(parseFontAxes(buf, 'ttf')).toEqual([]);
+  });
+
+  it('treats woff/woff2 as static — their tables are compressed, not parsed', () => {
+    const sfnt = buildVariableSfnt([['wght', 100, 400, 900]]);
+    expect(parseFontAxes(sfnt, 'woff2')).toEqual([]);
+    expect(parseFontAxes(sfnt, 'woff')).toEqual([]);
+  });
+
+  it('degrades to [] on a malformed file rather than throwing', () => {
+    expect(parseFontAxes(new Uint8Array([1, 2, 3]).buffer, 'ttf')).toEqual([]);
+  });
+});
+
+describe('variable-font helpers', () => {
+  const wght: FontAxis = { tag: 'wght', min: 100, default: 400, max: 900 };
+  const wdth: FontAxis = { tag: 'wdth', min: 75, default: 100, max: 100 };
+
+  it('isVariableFont reflects whether axes are present', () => {
+    expect(isVariableFont([wght])).toBe(true);
+    expect(isVariableFont([])).toBe(false);
+    expect(isVariableFont(undefined)).toBe(false);
+  });
+
+  it('maps wght→weight range and wdth→stretch range; ignores other axes', () => {
+    expect(variableFontDescriptors([wght, wdth])).toEqual({
+      weight: '100 900',
+      stretch: '75% 100%',
+    });
+    expect(variableFontDescriptors([{ tag: 'opsz', min: 8, default: 14, max: 144 }])).toEqual({});
+    expect(variableFontDescriptors(undefined)).toEqual({});
+  });
+});

src/core/font-asset.ts

@@ -0,0 +1,265 @@
+/**
+ * FontAsset — a user-provided font face stored once and reused across themes and
+ * snippets (docs/chart-theming-scope.md §4 → User font upload).
+ *
+ * Portable core: record shape, factory, format detection, family-name
+ * derivation, and variable-font (`fvar`) parsing. The raw bytes
+ * live in `data` (an `ArrayBuffer`, stored verbatim by IndexedDB's structured
+ * clone — `ArrayBuffer` is a platform global, not a DOM API, so this stays
+ * browser-free). Registering the face as a `FontFace` and persisting it are
+ * infrastructure concerns; this module never touches `document`/`indexedDB`.
+ *
+ * A font is referenced by its `family` — the CSS family name written into a
+ * config's `font`/`*Font` slots — exactly as a dataset is referenced by name.
+ * The family is unique across the font library (case-insensitive, via `naming`),
+ * so a single family maps to a single uploaded face.
+ *
+ * `source` records provenance so later tiers reuse this one store: `file` is an
+ * uploaded file; `google` (reserved) is a face fetched and cached from the
+ * keyless Google Fonts catalog. Either way the bytes live in `data`, so export
+ * and the SVG embed treat every FontAsset the same regardless of source.
+ */
+
+/** A FontAsset record's schema version (read-time migration target). */
+export const CURRENT_FONT_VERSION = 1;
+
+/** Supported font container formats, keyed by file extension. */
+export type FontFormat = 'woff2' | 'woff' | 'ttf' | 'otf';
+
+/**
+ * One variation axis of a variable font (the OpenType `fvar` table), e.g.
+ * `wght` (weight) or `wdth` (width). Stored on a FontAsset so registration can
+ * declare the right `FontFace` ranges and the UI can flag the face as variable.
+ */
+export interface FontAxis {
+  /** 4-char OpenType axis tag, e.g. `wght`, `wdth`, `opsz`. */
+  tag: string;
+  min: number;
+  default: number;
+  max: number;
+}
+
+/** Where an uploaded face came from. Only `file` ships today; `google` is reserved. */
+export type FontSource = 'file' | 'google';
+
+/**
+ * Soft cap on a single uploaded face, in bytes. A Latin display woff2 is tens of
+ * KB; this 10 MB ceiling admits a full TrueType (even a small CJK face) while
+ * refusing a pathological upload that would bloat IndexedDB. Enforced at the
+ * upload boundary (`services/fonts`), surfaced as a clear error — not silently here.
+ */
+export const MAX_FONT_BYTES = 10 * 1024 * 1024;
+
+export interface FontAsset {
+  /** Unique numeric identifier (IndexedDB key). */
+  id: number;
+  /** Record schema version, for read-time migration. */
+  version: number;
+  /** Unique, human-readable CSS family name — the key configs reference. */
+  family: string;
+  /** The raw font-file bytes (stored verbatim; registered as a `FontFace`). */
+  data: ArrayBuffer;
+  /** Container format, from the file extension. */
+  format: FontFormat;
+  /** Original file name, kept for display and provenance. */
+  fileName: string;
+  /** Provenance: an uploaded file, or (reserved) a cached Google face. */
+  source: FontSource;
+  /**
+   * Variation axes if this is a variable font (parsed from `fvar` at upload),
+   * else absent/empty for a static face. Drives the `FontFace` weight/width
+   * ranges so one variable file serves the whole weight range.
+   */
+  axes?: FontAxis[];
+  /** Byte length of `data`. */
+  size: number;
+  /** ISO timestamp — when first added. */
+  created: string;
+  /** ISO timestamp — when last changed (e.g. renamed). */
+  modified: string;
+}
+
+/** Map each accepted file extension to its container format. */
+const EXTENSION_FORMAT: Record<string, FontFormat> = {
+  woff2: 'woff2',
+  woff: 'woff',
+  ttf: 'ttf',
+  otf: 'otf',
+};
+
+/**
+ * The container format for a file name, or `null` when the extension isn't a
+ * supported font format. Extension-based on purpose: font MIME types are
+ * inconsistent across browsers and OSes (`.ttf` arrives as `font/ttf`,
+ * `application/x-font-ttf`, `application/octet-stream`, or `''`), so the
+ * extension is the reliable signal.
+ */
+export function detectFontFormat(fileName: string): FontFormat | null {
+  const ext = fileName.split('.').pop()?.toLowerCase() ?? '';
+  return EXTENSION_FORMAT[ext] ?? null;
+}
+
+/**
+ * A clean default family name derived from a file name: drop the extension,
+ * turn `-`/`_` separators into spaces, collapse whitespace. `Inter-Regular.ttf`
+ * → `Inter Regular`. A starting point the user can rename; never authoritative.
+ */
+export function familyNameFromFileName(fileName: string): string {
+  const base = fileName.replace(/\.[^.]+$/, '');
+  const cleaned = base.replace(/[-_]+/g, ' ').replace(/\s+/g, ' ').trim();
+  return cleaned || 'Custom font';
+}
+
+export interface CreateFontAssetOptions {
+  /** The CSS family name (uniqueness enforced upstream — see `naming`). */
+  family: string;
+  /** The raw font-file bytes. */
+  data: ArrayBuffer;
+  /** Container format. */
+  format: FontFormat;
+  /** Original file name. */
+  fileName: string;
+  /** Provenance; defaults to `file`. */
+  source?: FontSource;
+  /** Variation axes (parsed from `fvar`) when the file is a variable font. */
+  axes?: FontAxis[];
+  /** Clock injection for deterministic tests; defaults to the current time. */
+  now?: Date;
+  /** Id injection for deterministic tests; defaults to `Date.now()`. */
+  id?: number;
+}
+
+/**
+ * Build a FontAsset: stamps version/timestamps and computes `size`. The default
+ * id is provisional — the store's id authority reassigns it on insert (same
+ * contract as `createDataset`/`createCustomTheme`).
+ */
+export function createFontAsset(options: CreateFontAssetOptions): FontAsset {
+  const iso = (options.now ?? new Date()).toISOString();
+  return {
+    id: options.id ?? Date.now(),
+    version: CURRENT_FONT_VERSION,
+    family: options.family,
+    data: options.data,
+    format: options.format,
+    fileName: options.fileName,
+    source: options.source ?? 'file',
+    ...(options.axes && options.axes.length > 0 ? { axes: options.axes } : {}),
+    size: options.data.byteLength,
+    created: iso,
+    modified: iso,
+  };
+}
+
+/**
+ * The CSS font stack written into a config for an uploaded family: the quoted
+ * family plus a generic fallback. The upload tier doesn't know a face's category
+ * (serif/sans/mono), so `sans-serif` is the neutral last resort if the face
+ * itself fails to load. Mirrors the roster's stacks in `THEME_FONT_OPTIONS`.
+ */
+export function fontFamilyStack(family: string): string {
+  return `"${family}", sans-serif`;
+}
+
+// --- Variable-font (fvar) parsing ------------------------------------------
+
+const SFNT_TTF = 0x00010000; // TrueType outlines
+const SFNT_OTTO = 0x4f54544f; // 'OTTO' — CFF (OpenType) outlines
+const SFNT_TRUE = 0x74727565; // 'true' — legacy TrueType
+
+/** Read a 4-byte tag as ASCII. */
+function readTag(view: DataView, offset: number): string {
+  return String.fromCharCode(
+    view.getUint8(offset),
+    view.getUint8(offset + 1),
+    view.getUint8(offset + 2),
+    view.getUint8(offset + 3),
+  );
+}
+
+/** OpenType 16.16 fixed-point (big-endian) → number. */
+function readFixed(view: DataView, offset: number): number {
+  return view.getInt32(offset) / 65536;
+}
+
+/**
+ * Parse a font's variation axes from its OpenType `fvar` table, returning `[]`
+ * for a static font (or one we can't read).
+ *
+ * Only **uncompressed SFNT** (`ttf`/`otf`) is read directly. `woff`/`woff2` wrap
+ * the table data in zlib/brotli compression we don't unpack, so those uploads
+ * are treated as static (registered pinned) — a deliberate, safe fallback rather
+ * than a failure. A malformed file likewise degrades to `[]`, not a throw: axis
+ * detection is an enhancement, and the face still registers and renders.
+ */
+export function parseFontAxes(data: ArrayBuffer, format: FontFormat): FontAxis[] {
+  if (format !== 'ttf' && format !== 'otf') return [];
+  try {
+    if (data.byteLength < 12) return [];
+    const view = new DataView(data);
+    const sfnt = view.getUint32(0);
+    if (sfnt !== SFNT_TTF && sfnt !== SFNT_OTTO && sfnt !== SFNT_TRUE) return [];
+
+    const numTables = view.getUint16(4);
+    let fvar = -1;
+    for (let i = 0; i < numTables; i++) {
+      const rec = 12 + i * 16; // table directory: 16-byte records after the 12-byte header
+      if (readTag(view, rec) === 'fvar') {
+        fvar = view.getUint32(rec + 8); // record: tag, checksum, offset, length
+        break;
+      }
+    }
+    if (fvar < 0) return [];
+
+    // fvar header: version(4) axesArrayOffset(2) reserved(2) axisCount(2)
+    //              axisSize(2) instanceCount(2) instanceSize(2)
+    const axesArrayOffset = view.getUint16(fvar + 4);
+    const axisCount = view.getUint16(fvar + 8);
+    const axisSize = view.getUint16(fvar + 10);
+    const base = fvar + axesArrayOffset;
+
+    const axes: FontAxis[] = [];
+    for (let i = 0; i < axisCount; i++) {
+      const a = base + i * axisSize; // axis record: tag(4) min(Fixed) default(Fixed) max(Fixed) flags(2) nameID(2)
+      axes.push({
+        tag: readTag(view, a),
+        min: readFixed(view, a + 4),
+        default: readFixed(view, a + 8),
+        max: readFixed(view, a + 12),
+      });
+    }
+    return axes;
+  } catch {
+    return [];
+  }
+}
+
+/** Whether a face carries variation axes (i.e. it's a variable font). */
+export function isVariableFont(axes: FontAxis[] | undefined): boolean {
+  return (axes?.length ?? 0) > 0;
+}
+
+/**
+ * `FontFace` descriptor ranges for a variable font so the browser resolves a
+ * requested weight/width against the file's axes — one variable file then serves
+ * the whole weight range, driven by the Type panel's weight controls. Empty for
+ * a static font.
+ *
+ * Only `wght`→`weight` and `wdth`→`stretch` are mapped: those flow through the
+ * CSS font shorthand Vega builds for canvas/SVG text, so they actually take
+ * effect at render. Other axes (optical size, grade, custom) have no text-render
+ * hook in Vega and would only pin at the registered default, so we don't pretend
+ * to expose them. Declaring the `wdth` range also defaults the face to normal
+ * width (100%) instead of a variable font's possibly-condensed default instance.
+ */
+export function variableFontDescriptors(axes: FontAxis[] | undefined): {
+  weight?: string;
+  stretch?: string;
+} {
+  const out: { weight?: string; stretch?: string } = {};
+  const wght = axes?.find((a) => a.tag === 'wght');
+  if (wght) out.weight = `${wght.min} ${wght.max}`;
+  const wdth = axes?.find((a) => a.tag === 'wdth');
+  if (wdth) out.stretch = `${wdth.min}% ${wdth.max}%`;
+  return out;
+}