diff --git a/docs/architecture/01-state-and-stores.md b/docs/architecture/01-state-and-stores.md
index be7789b..4cf626c 100644
--- a/docs/architecture/01-state-and-stores.md
+++ b/docs/architecture/01-state-and-stores.md
@@ -351,6 +351,23 @@ test('deleting the active snippet selects the next one', () => {
 > action. Local, throwaway UI state (a dropdown's open flag) may stay in component
 > `useState`; anything another component reads belongs in a store behind an action.
 
+### Change detection: reference identity, not serialization
+
+Because every action replaces a state object via spread (never mutates it),
+"has this changed since X" is **reference identity** against the object captured
+at X. The Chart Builder's dataset switch keeps the exact config `init` produced
+(`initialConfig`) and asks `config === initialConfig` to tell an untouched
+opening default from built-on work. Field-level comparison against the source
+record (the Theme Builder's draft-dirty selector) is the equivalent for forms
+seeded from a saved record.
+
+> Rule: never detect change by serializing and comparing
+> (`JSON.stringify(a) === JSON.stringify(b)`) — it silently depends on key
+> order, costs proportionally to state size, and a store that replaces objects
+> immutably already has a cheaper, exact signal. The one sanctioned
+> serialization is the modal coordinator's unsaved-change **snapshot**
+> (architecture 03), where a cross-store, store-agnostic baseline is the point.
+
 ---
 
 ## 5. Effects: Persistence and External Sync
@@ -518,3 +535,5 @@ reset: () => set({ snippets: [], activeSnippetId: null, draftSpec: '' });
 - Don't touch IndexedDB/`localStorage`/URL adapters from components.
 - Don't thread global state down through props; don't pass per-instance or
   presentational data via store imports.
+- Don't detect change by serialize-and-compare; immutable replacement makes
+  reference identity the exact, cheap signal (§4 → Change detection).
diff --git a/docs/architecture/02-persistence.md b/docs/architecture/02-persistence.md
index 50fb4d9..02ea39a 100644
--- a/docs/architecture/02-persistence.md
+++ b/docs/architecture/02-persistence.md
@@ -469,7 +469,8 @@ single transaction spanning many records across stores**. So a bulk operation li
 the **service boundary** instead: write the new records to IndexedDB first, tracking what
 succeeded, and only on full success commit to the Zustand stores. On any write failure
 (typically `QuotaExceededError`) it **rolls back best-effort** — deletes the records written
-so far (`Promise.allSettled`) and removes any datasets already added to the store — so the
+so far (`Promise.allSettled`) and removes any datasets and custom themes already added to
+their stores (their persistence subscribers propagate the removals to IDB) — so the
 spec §08 "no partial import is committed" contract holds and the user gets an actionable
 "storage full, delete and retry" message.
 
diff --git a/docs/architecture/04-routing-and-events.md b/docs/architecture/04-routing-and-events.md
index 3b14dd3..1c4becc 100644
--- a/docs/architecture/04-routing-and-events.md
+++ b/docs/architecture/04-routing-and-events.md
@@ -33,18 +33,22 @@ Zustand stores. Components never read `location.hash` or attach
 
 The hash is the serialized view. Astrolabe's forms:
 
-| State                       | Hash                           |
-| --------------------------- | ------------------------------ |
-| Default snippets view       | _(empty / absent)_             |
-| A selected snippet          | `#snippet-<id>`                |
-| Datasets manager (list)     | `#datasets`                    |
-| A specific dataset          | `#datasets/dataset-<id>`       |
-| New-dataset form            | `#datasets/new`                |
-| Chart Builder for a dataset | `#datasets/dataset-<id>/build` |
+| State                          | Hash                           |
+| ------------------------------ | ------------------------------ |
+| Default snippets view          | _(empty / absent)_             |
+| A selected snippet             | `#snippet-<id>`                |
+| Datasets manager (list)        | `#datasets`                    |
+| A specific dataset             | `#datasets/dataset-<id>`       |
+| New-dataset form               | `#datasets/new`                |
+| Chart Builder for a dataset    | `#datasets/dataset-<id>/build` |
+| Chart Builder, no dataset open | `#build`                       |
 
 Snippet `id` is an opaque string; dataset `id` is the numeric dataset id
 rendered as a decimal string. The hash is the **only** persisted view-routing
-state — there is no in-memory "current route" that can drift from it.
+state — there is no in-memory "current route" that can drift from it. `#build`
+serializes the builder's no-datasets state only: an un-targeted builder open
+picks a dataset itself when any exist, so the derived view immediately
+self-corrects to the `dataset-build` form.
 
 ### 1.2 The adapter: `infrastructure/url-hash.ts`
 
@@ -62,7 +66,8 @@ export type ViewState =
   | { kind: 'datasets' } // #datasets
   | { kind: 'dataset'; datasetId: number } // #datasets/dataset-<id>
   | { kind: 'dataset-new' } // #datasets/new
-  | { kind: 'dataset-build'; datasetId: number }; // .../build
+  | { kind: 'dataset-build'; datasetId: number } // .../build
+  | { kind: 'build' }; // #build — builder, no dataset loaded
 
 export function parseHash(rawHash: string): ViewState {
   const hash = rawHash.replace(/^#/, '');
@@ -71,6 +76,8 @@ export function parseHash(rawHash: string): ViewState {
   const snippet = /^snippet-(.+)$/.exec(hash);
   if (snippet) return { kind: 'snippet', snippetId: snippet[1] };
 
+  if (hash === 'build') return { kind: 'build' };
+
   const parts = hash.split('/').filter(Boolean);
   if (parts[0] === 'datasets') {
     if (parts.length === 1) return { kind: 'datasets' };
@@ -100,6 +107,8 @@ export function serializeHash(view: ViewState): string {
       return '#datasets/new';
     case 'dataset-build':
       return `#datasets/dataset-${view.datasetId}/build`;
+    case 'build':
+      return '#build';
   }
 }
 
diff --git a/docs/architecture/10-interaction-and-feedback.md b/docs/architecture/10-interaction-and-feedback.md
index 259b76a..6b95b0a 100644
--- a/docs/architecture/10-interaction-and-feedback.md
+++ b/docs/architecture/10-interaction-and-feedback.md
@@ -445,7 +445,14 @@ refocuses the trigger; outside press closes; open lands focus on the selected op
 Arrow/Home/End rove). The selected option carries `aria-current` and a visible ✓, never
 colour alone. The same control doubles as an **action picker** (no `value`; e.g. "Add field
 to which channel?"). A custom `triggerClassName` _replaces_ the default trigger styling, so
-chip-styled triggers (the pill's type chip, the shelf's field chips) stay chips.
+chip-styled triggers (the pill's type chip, the shelf's field chips) stay chips; the default
+trigger sits on the **32px compact control scale** (arch 09 §6), like the sort trigger and
+search input it shares surfaces with. A long option list can carry **group separators**: an
+option's `dividerBefore` draws a `role="presentation"` rule above it — purely visual, never
+in the keyboard order, never a heading. **The boundary is set where the list is built**: the
+module that decides the option order marks the divider-carrying option (e.g.
+`chartThemeOptions` stamps the first preset); a consumer must never recompute a group
+boundary by index arithmetic, which silently misplaces when the producer's ordering changes.
 The single-open registry means **disclosures cannot nest**: a SelectControl inside a
 settings popover would close — and unmount — its own parent on open. A control that needs
 its own popover sits beside the gear in the pane header, never inside the panel.