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/components/chat/MarkdownRenderer.tsx

@@ -2,7 +2,7 @@ import React from 'react';
 import { lazyWithChunkRecovery } from '@/lib/chunkLoadRecovery';
 import { isMobileSurfaceRuntime } from '@/lib/runtimeSurface';
 import { cn } from '@/lib/utils';
-import { loadMarkdownRendererModule } from './markdownRendererLoader';
+import { getLoadedMarkdownRendererModule, loadMarkdownRendererModule } from './markdownRendererLoader';
 
 // Thin lazy wrapper around the MarkdownRenderer implementation.
 // The full implementation (marked + Shiki highlighting + KaTeX + morphdom
@@ -41,21 +41,29 @@ const MobileMarkdownFallback = (props: { content?: unknown; className?: unknown;
   );
 };
 
-export const MarkdownRenderer: React.FC<React.ComponentPropsWithoutRef<typeof MarkdownRendererLazy>> = (props) => (
-  <React.Suspense fallback={<MobileMarkdownFallback {...props} />}>
-    <MarkdownRendererLazy {...props} />
-  </React.Suspense>
-);
+export const MarkdownRenderer: React.FC<React.ComponentPropsWithoutRef<typeof MarkdownRendererLazy>> = (props) => {
+  const loaded = getLoadedMarkdownRendererModule();
+  if (loaded) return <loaded.MarkdownRenderer {...props} />;
+  return (
+    <React.Suspense fallback={<MobileMarkdownFallback {...props} />}>
+      <MarkdownRendererLazy {...props} />
+    </React.Suspense>
+  );
+};
 
 type SimpleMarkdownRendererProps = React.ComponentPropsWithoutRef<typeof SimpleMarkdownRendererLazy> & {
   fallbackContent?: React.ReactNode;
 };
 
-export const SimpleMarkdownRenderer: React.FC<SimpleMarkdownRendererProps> = ({ fallbackContent, ...props }) => (
-  <React.Suspense fallback={fallbackContent ?? <MobileMarkdownFallback {...props} />}>
-    <SimpleMarkdownRendererLazy {...props} />
-  </React.Suspense>
-);
+export const SimpleMarkdownRenderer: React.FC<SimpleMarkdownRendererProps> = ({ fallbackContent, ...props }) => {
+  const loaded = getLoadedMarkdownRendererModule();
+  if (loaded) return <loaded.SimpleMarkdownRenderer {...props} />;
+  return (
+    <React.Suspense fallback={fallbackContent ?? <MobileMarkdownFallback {...props} />}>
+      <SimpleMarkdownRendererLazy {...props} />
+    </React.Suspense>
+  );
+};
 
 export const MarkdownImageGallery: React.FC<React.ComponentPropsWithoutRef<typeof MarkdownImageGalleryLazy>> = (props) => (
   <React.Suspense fallback={null}>