Implement M1 authoring loop: library, editor, live preview, persistence

M1 MVP from docs/IMPLEMENTATION-PLAN.md, with the /alignment pass applied.

- core: Snippet model + factory; prepareSpecForRender (copy-not-mutate); per-theme Vega chart config
- state/orchestration: SnippetStore with debounced auto-save; IndexedDB adapter + read-time migration; startup hydration + write-through persistence
- ui: SnippetLibrary, SpecEditor (Monaco edcore.main — full editor features, JSON-only languages), LivePreview
- build: Monaco/Vega manual chunks; raised PWA precache ceiling
- alignment: flush a valid draft on snippet switch (+regression tests); TODO breadcrumbs for the preview render race and the window.confirm delete
- housekeeping: gitignore .claude/projects/

src/core/rendering.ts

@@ -0,0 +1,47 @@
+/**
+ * Rendering contract — pure spec preparation (spec §04 → Rendering Contract).
+ *
+ * Portable core: no browser APIs, no React, no vega-embed. `prepareSpecForRender`
+ * is the single transform that sits between "parsed spec the user authored" and
+ * "spec the preview actually embeds" (see docs/architecture/05). It performs two
+ * deterministic steps, in order, **on a deep copy** so the user's stored spec is
+ * never mutated by rendering:
+ *
+ *   1. Dataset reference resolution  — arrives in M3 (no-op here).
+ *   2. Fit-mode sizing               — arrives in M2 (no-op here).
+ *
+ * In M1 it is an identity transform over a copy: it establishes the
+ * copy-not-mutate invariant and the call site the renderer depends on, so M2/M3
+ * can fill in the steps without the preview pipeline changing shape.
+ */
+
+/** Preview sizing modes (spec §04 → Fit / Sizing Modes). `default` = Original. */
+export type FitMode = 'default' | 'width' | 'height' | 'full';
+
+export interface PrepareOptions {
+  /** Active fit mode. Applied in M2; ignored in M1. */
+  fitMode?: FitMode;
+}
+
+/**
+ * Escape `.`/`[`/`]` so Vega-Lite treats a string as a literal field name rather
+ * than a nested-property accessor (docs/architecture/05 §4). Used wherever
+ * Astrolabe *constructs* a `field:` from a data-derived column name (chart
+ * builder, M4); hand-authored specs are the user's responsibility.
+ */
+export function escapeVegaField(name: string): string {
+  return name.replace(/([.[\]])/g, '\\$1');
+}
+
+/**
+ * Transform the authored spec into the spec to embed. Operates on a deep copy
+ * and returns it; the input is never mutated.
+ */
+export function prepareSpecForRender<T>(spec: T, _options: PrepareOptions = {}): T {
+  const copy = structuredClone(spec);
+
+  // M3: resolveDatasetRefs(copy, datasets)
+  // M2: applyFitMode(copy, options.fitMode)
+
+  return copy;
+}