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/DOCUMENTATION.md

@@ -413,6 +413,49 @@ The global stream can omit a directory for a session-addressed event. Resolve it
 
 ## Selector hygiene
 
+### Runtime context versus directory context
+
+`SyncProvider` publishes two contexts. `SyncRuntimeContext` (`useSyncRuntime()`)
+holds the child-store manager, message loader, SDK, runtime key, and a
+subscribable `currentDirectory` source; its value changes only on runtime
+reconfiguration. `SyncContext` (`useSyncSystem()` / `useSync()`) adds the
+current directory string, so every consumer re-renders on each directory
+switch.
+
+A hook that takes an explicit directory, or needs only runtime fields, must
+read `useSyncRuntime()`. `useDirectoryStore(directory)` reads the current
+directory through `runtime.currentDirectory` with `useSyncExternalStore`, so a
+consumer that passes its own directory gets a constant snapshot and is not
+re-rendered by a cross-project switch. This is what keeps sidebar rows
+(permissions, question counts, session lookups) out of the switch commit: a
+row must not pay for the chat changing directory.
+
+### Session switch commit
+
+The sidebar click publishes `currentSessionId`/`currentSessionDirectory`
+synchronously, and the message fetch starts before that publication so the
+request is on the wire while React renders. `ChatContainer` consumes a
+`useDeferredValue` copy of the selection: the first commit paints the cheap
+reactions (active row, URL, tabs) and the timeline for the new session renders
+in a transition behind it. Selection *policy* inside `ChatContainer` (auto-
+opening a draft when nothing is selected) reads the live store value, because
+the deferred one still names the previous session for one commit.
+
+The timeline's first paint for a session is atomic. `ChatContainer` owns a
+`TimelineRevealGate` per session key (`components/chat/timelineRevealGate.ts`):
+a markdown renderer whose first paint is provisional (blocks not yet in the
+settled cache, so code is unhighlighted) takes a hold in its layout effect,
+and the timeline root stays at opacity 0 until every hold releases, capped at
+250ms, then fades in once as a whole. A warm switch takes no holds and reveals
+in the same frame. The gate stops accepting holds after the opening commit so
+rows mounting during scroll never hide the timeline. Once the lazy markdown
+module has loaded, `MarkdownRenderer` mounts it synchronously instead of
+through `Suspense`: a suspended boundary shows its fallback for a tick and
+React then throttles later-resolving boundaries by ~300ms, which staggered
+user and assistant text on a cold open.
+
+`bun run profile:switch` measures both moments; see `scripts/perf/DOCUMENTATION.md`.
+
 Select leaf values, not containers:
 
 ```typescript