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/layout/SessionTabsStrip.tsx

@@ -159,8 +159,11 @@ const SessionTabItem: React.FC<{
                   }}
                   data-controls-open={overlayVisible ? 'true' : 'false'}
                   className={cn(
+                    // No color transition: activation must snap. A crossfade
+                    // here reads as the switch itself being slow, since the
+                    // old and new tab trade colors over several frames right
+                    // after the click.
                     'session-tab group/session-tab relative flex h-7 w-full min-w-0 select-none items-center rounded-md px-2',
-                    'transition-colors duration-75',
                     isActive
                       ? 'bg-interactive-selection'
                       : cn(
@@ -180,8 +183,15 @@ const SessionTabItem: React.FC<{
                       !suppressControls && 'session-tab-title',
                     )}
                     >
+                      {/* Same box as the active content the header renders
+                          (a centered column with a block title), so the
+                          title sits at the same height before and after
+                          activation and does not jump when the tab swaps
+                          its content. */}
                       {isActive ? children : (
-                        <span className="text-[13px] font-medium leading-4">{title}</span>
+                        <div className="flex min-w-0 flex-col justify-center">
+                          <span className="block max-w-full overflow-hidden whitespace-nowrap text-[13px] font-medium leading-4">{title}</span>
+                        </div>
                       )}
                     </div>
                     {showDot ? (