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/sync/session-ui-store.ts

@@ -954,6 +954,16 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
       )
       : null
 
+    // Start the message fetch before publishing the selection. React flushes
+    // the discrete-event render in a microtask queued by `set`, so a fetch
+    // started after it would only leave the browser once that whole render
+    // finished. Started first, the request is on the wire while the render
+    // runs. Fire-and-forget: any transient failure is retried by the reactive
+    // path in ChatContainer.
+    if (id) {
+      void fetchMessagesForSession(id, resolvedDir)
+    }
+
     // Set the directory together with the session id so chat hooks read the
     // same child store that send/SSE events will update during startup races.
     set({
@@ -970,13 +980,6 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
       persistLastActiveSession(key, { sessionId: id, directory: rememberedDir })
     }
 
-    // Kick off the message fetch on the same tick, before React commits the
-    // state change and fires ChatContainer.useEffect. The fetch is
-    // fire-and-forget — any transient failure gets retried by the reactive path.
-    if (id) {
-      void fetchMessagesForSession(id, resolvedDir)
-    }
-
     try {
       if (resolvedDir && directoryState.currentDirectory !== resolvedDir) {
         directoryState.setDirectory(resolvedDir, { showOverlay: false })