fix(perf,a11y): selectAtom for unread subscriptions, semantic headings, Perf-5 binary search

Perf-3: Replace raw roomToUnreadAtom subscription in Home, Direct, Space with
  selectAtom-derived Set<string> — components now only re-render when rooms
  gain/lose unread presence, not on every notification count update
Perf-5: RoomTimeline eventRenderer now uses binary search on precomputed
  timelineSegments instead of O(N×T) linear scan per visible message
A11y L-1: Add as=h2 semantic heading to Home, Direct, Inbox, Space page nav
  titles so screen readers announce page sections correctly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/pages/client/home/Home.tsx

@@ -17,6 +17,7 @@ import {
 } from 'folds';
 import { useVirtualizer } from '@tanstack/react-virtual';
 import { useAtom, useAtomValue } from 'jotai';
+import { selectAtom } from 'jotai/utils';
 import FocusTrap from 'focus-trap-react';
 import { factoryRoomIdByActivity, factoryRoomIdByAtoZ } from '../../../utils/sort';
 import {
@@ -116,7 +117,7 @@ function HomeHeader() {
       <PageNavHeader>
         <Box alignItems="Center" grow="Yes" gap="300">
           <Box grow="Yes">
-            <Text size="H4" truncate>
+            <Text as="h2" size="H4" truncate>
               Home
             </Text>
           </Box>
@@ -200,7 +201,26 @@ export function Home() {
   const scrollRef = useRef<HTMLDivElement>(null);
   const rooms = useHomeRooms();
   const notificationPreferences = useRoomsNotificationPreferencesContext();
-  const roomToUnread = useAtomValue(roomToUnreadAtom);
+  // Perf-3: only re-render when the set of rooms WITH unread changes, not on count updates
+  const roomsWithUnreadSet = useAtomValue(
+    useMemo(
+      () =>
+        selectAtom(
+          roomToUnreadAtom,
+          (rtu) => {
+            const s = new Set<string>();
+            for (const id of rtu.keys()) s.add(id);
+            return s;
+          },
+          (a, b) => {
+            if (a.size !== b.size) return false;
+            for (const id of a) if (!b.has(id)) return false;
+            return true;
+          }
+        ),
+      []
+    )
+  );
   const navigate = useNavigate();
 
   const selectedRoomId = useSelectedRoom();
@@ -216,10 +236,10 @@ export function Home() {
         : factoryRoomIdByAtoZ(mx)
     );
     if (closedCategories.has(DEFAULT_CATEGORY_ID)) {
-      return items.filter((rId) => roomToUnread.has(rId) || rId === selectedRoomId);
+      return items.filter((rId) => roomsWithUnreadSet.has(rId) || rId === selectedRoomId);
     }
     return items;
-  }, [mx, rooms, closedCategories, roomToUnread, selectedRoomId]);
+  }, [mx, rooms, closedCategories, roomsWithUnreadSet, selectedRoomId]);
 
   const virtualizer = useVirtualizer({
     count: sortedRooms.length,