perf(chat): make session switching feel instant

Switching sessions ran as one synchronous commit: sidebar highlight, URL,
a full timeline remount with markdown re-parse, and around nine requests,
so nothing changed on screen for 150-250ms after the click.

- ChatContainer swaps the timeline on a deferred copy of the selection, so
  the active row, URL, and tab commit first and the timeline renders behind
  them; selection policy keeps reading the live store value.
- The message fetch starts before the selection is published.
- Sidebar rows stop re-rendering on a project switch: directory-scoped sync
  hooks read the runtime context and a subscribable current-directory source
  instead of the directory-bearing context; the grouping builder reads git
  branches through a ref and section caches key the branches they use;
  descendant ids are keyed by content. Rows per switch went from 73 to 8.
- Markdown skips the async re-render when the settled cached blocks are
  already painted, and mounts synchronously once its lazy module is loaded;
  the module is preloaded at boot.
- A timeline reveal gate holds a freshly opened session at opacity 0 while
  any provisional markdown paint catches up (250ms cap), then fades the whole
  timeline in once, so text, tools, and recap appear together.
- Switch fan-out trimmed: knowledge summary deduped, MCP status refreshed only
  when stale, non-repo directories cached by the git repo check, OpenChamber
  defaults cached briefly, agent memory reused for the same project, goal
  text cached, PWA manifest rebuilt after the switch settles.
- Header tabs snap into the active state and keep the title at the same
  height in both states.
- Prefetch on row press; composer focus moved off the commit.

`bun run profile:switch` records ack/content latency, longest task, and
requests per switch, cold and warm, and compares runs against a baseline.
Measured warm switch: ack 228ms to about 40-60ms, content 228ms to about
100-120ms.

packages/ui/src/hooks/useSessionGoal.ts

@@ -22,6 +22,9 @@ export function useSessionGoal(sessionId: string, directory?: string): SessionGo
   };
 }
 
+const OBJECTIVE_CONTENT_CACHE_MAX = 64;
+const objectiveContentByFetchKey = new Map<string, Promise<string | null>>();
+
 // Effective objective text for display. Inline goals return the metadata
 // text directly; file-backed goals fetch the server-side file once per
 // goal edit (keyed by id + updatedAt). Display-only: a failed fetch yields
@@ -37,7 +40,18 @@ export function useGoalObjectiveContent(sessionId: string, goal: SessionGoalPayl
       return undefined;
     }
     let alive = true;
-    void fetchGoalObjectiveContent(sessionId).then((content) => {
+    // The key already names the goal edit, so a remount (every session switch
+    // remounts the strip) reuses the text instead of fetching the file again.
+    let request = objectiveContentByFetchKey.get(fetchKey);
+    if (!request) {
+      request = fetchGoalObjectiveContent(sessionId);
+      objectiveContentByFetchKey.set(fetchKey, request);
+      if (objectiveContentByFetchKey.size > OBJECTIVE_CONTENT_CACHE_MAX) {
+        const oldest = objectiveContentByFetchKey.keys().next().value;
+        if (oldest !== undefined) objectiveContentByFetchKey.delete(oldest);
+      }
+    }
+    void request.then((content) => {
       if (alive) setFetched(content);
     });
     return () => {