Add snippet metadata panel, duplicate, and immediate-load preview (M4.5)

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

@@ -333,6 +333,28 @@ useSettingsStore.subscribe((s, prev) => {
 });
 ```
 
+### Implemented policy: what renders immediately vs. debounced
+
+> The service above is a **sketch**; the shipped renderer lives inline in
+> `LivePreview.tsx` (one `setTimeout` whose delay is computed per change) and
+> subscribes to the stores via hooks rather than startup subscribers. When it is
+> extracted into a service, preserve this policy.
+
+The debounce exists to stay out of the way **while typing** — nothing else. So the
+delay is `0` (immediate) for everything except keystrokes (spec §03C):
+
+- **Immediate** — a _programmatic buffer load_ (`SnippetStore.bufferEpoch` changed:
+  select / create / duplicate / revert / hydrate) or a _Draft↔Published switch_
+  (`editorView` changed). These are the cases §03C names; the editor and preview
+  both key off `bufferEpoch` to tell a load from a keystroke.
+- **Debounced** — a keystroke (only `shownText` changed). This is the churn the
+  debounce protects against.
+
+Detect "this was a keystroke" by elimination: `shownText` changed but `bufferEpoch`
+and `editorView` did **not**. Fit-mode and theme changes currently fall through the
+debounce too (harmless; not typing) — flush them if instant feedback is wanted, but
+never debounce a load or a view switch.
+
 ### Busy indicator
 
 `setBusy(true/false)` toggles store state that the preview reads to overlay a
@@ -532,7 +554,7 @@ bookkeeping. Gate the observer to responsive modes (Original needs no re-fit).
 | View teardown | `view.finalize()` before each re-render and on unmount                               | the renderer's `RenderHandle`                   |
 | Theming       | Vega `Config` per UI theme, applied at embed time                                    | `chartConfigFor()` in `src/core/vega-themes.ts` |
 | Field names   | `escapeVegaField` on every data-derived `field:`                                     | `src/core/rendering.ts`                         |
-| Debounce      | `createDebouncedRenderer`, delay from `renderDebounce` setting                       | `src/app/services/debounced-renderer.ts`        |
+| Debounce      | Inline timer; `0` on buffer-load/view-switch, `renderDebounce` on keystroke (§5)     | `LivePreview.tsx` (service not yet extracted)   |
 | Spec prep     | `prepareSpecForRender` (pure, on a copy)                                             | `src/core/rendering.ts` (see _Live Preview_)    |
 | Errors        | One error field, cleared on success, empty = nothing                                 | `PreviewStore.error`                            |
 | Container fit | Inner host + frame (out-specify `.vega-embed`); resize via synthetic `window:resize` | §8 (`LivePreview` + `chart-renderer`)           |