fix(ui): close review findings from the switch-and-scroll integration

- Markdown DOM cache: key includes a content-length fingerprint so an
  edited or reverted part re-materializing under the same id cannot
  restore stale DOM, memoization uses scalar identities instead of the
  part object (store reducers recreate part objects on unrelated updates,
  which re-ran the async render pipeline for identical content), and a
  probe with a mismatched locale/directory no longer destroys the entry
  it failed to claim.
- Sidebar bootstrap: the layout-level sync owner only knows known
  directories, so expanded projects bootstrapped serialized at background
  priority. The visible collection now publishes a second, expansion-
  aware demand owner, restoring concurrent hydration for expanded
  projects and worktree groups.
- Settings: local changes still sitting in the debounce buffer are not
  yet tracked as mutations, so a settings GET racing the debounce window
  briefly reverted them; reconciled results now reapply the pending
  buffer.

packages/ui/src/components/chat/MarkdownRendererImpl.tsx

@@ -1122,17 +1122,25 @@ const MarkdownRendererImpl: React.FC<MarkdownRendererProps> = ({
     ? part
     : null;
   const runtimeKey = getRuntimeKey();
+  // Memoized on scalar identities, not the part object: sync-store reducers
+  // recreate part objects on unrelated updates, and an object-identity dep
+  // re-ran the async render pipeline for identical content.
+  const settledSessionID = settledPart?.sessionID;
+  const settledMessageID = settledPart?.messageID;
+  const settledPartID = settledPart?.id;
   const domCacheKey = React.useMemo<DetachedMarkdownDomKey | null>(() => {
     // Streaming, unfinished, oversized, and identity-less Markdown continues
     // through the normal rendering pipeline and never retains detached DOM.
-    if (isStreaming || !settledPart || content.length === 0 || content.length > MARKDOWN_DOM_CACHE_MAX_SOURCE_CHARS) return null;
+    if (isStreaming || !settledSessionID || !settledMessageID || !settledPartID || content.length === 0 || content.length > MARKDOWN_DOM_CACHE_MAX_SOURCE_CHARS) return null;
+    // content.length is a cheap fingerprint: an edited or reverted part that
+    // re-materializes under the same id must not restore the old DOM.
     return {
-      scope: `${runtimeKey}\0${settledPart.sessionID}`,
-      id: `${settledPart.messageID}\0${settledPart.id}\0${imageMode}`,
+      scope: `${runtimeKey}\0${settledSessionID}`,
+      id: `${settledMessageID}\0${settledPartID}\0${imageMode}\0${content.length}`,
       locale,
       directory: effectiveDirectory,
     };
-  }, [content.length, effectiveDirectory, imageMode, isStreaming, locale, runtimeKey, settledPart]);
+  }, [content.length, effectiveDirectory, imageMode, isStreaming, locale, runtimeKey, settledSessionID, settledMessageID, settledPartID]);
   // Identity for the fade-in wrapper: a new part/message restarts the animation.
   const fadeKey = `markdown-${part?.id ? `part-${part.id}` : `message-${messageId}`}`;
 

packages/ui/src/components/chat/markdown/detachedMarkdownDomCache.ts

@@ -77,10 +77,14 @@ export class DetachedMarkdownDomCache {
     const entry = session.get(entryKey);
     if (entry === undefined) return null;
 
+    // A mismatched probe (different locale or directory for the same part)
+    // must not destroy the entry — the matching renderer may still come for
+    // it. Only a real hit transfers ownership out of the cache.
+    if (entry.locale !== key.locale || entry.directory !== key.directory) return null;
+
     // A fragment is a move-only resource; taking it removes cache ownership.
     session.delete(entryKey);
     if (session.size === 0) this.sessions.delete(sessionKey);
-    if (entry.locale !== key.locale || entry.directory !== key.directory) return null;
     return entry.fragment;
   }