Chart theming: selectable chart theme + spec↔config merge/extract

docs/architecture/01-state-and-stores.md

@@ -376,7 +376,22 @@ useAppStore.subscribe((s, prev) => {
 
 The store stays DOM-free; the adapter (the `applyTheme` subscriber) lives at the edge.
 
-### Debounced auto-save of the draft spec
+### Adding a persisted UI preference (the established chain)
+
+A small global preference (preview fit mode, chart theme) follows one chain — five touch
+points, in order:
+
+1. `core/settings.ts` — field on `UserSettings` + `defaultSettings()` + `loadSettings`
+   validation (an unknown stored value falls back to the default, never breaks the app).
+2. `infrastructure/settings-store.ts` — slice loader/saver pair using the per-slice
+   write-through merge (doc 02 §5), so other writers of the shared record survive.
+3. `stores/AppStore.ts` — field + setter (the store stays browser-free).
+4. `orchestration/preferences.ts` — `initX()` hydrates the store from the adapter;
+   `wireX()` subscribes store → adapter.
+5. `main.tsx` — call both before `createRoot().render` (the store is the single source
+   of truth from first paint).
+
+The UI control only calls the AppStore setter; persistence follows from the subscriber.
 
 The Monaco editor writes every keystroke into `draftSpec`. We do **not** persist on
 every keystroke. A startup subscriber observes the draft and debounces the expensive