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/space/Space.tsx

@@ -7,6 +7,7 @@ import React, {
   useState,
 } from 'react';
 import { useAtom, useAtomValue } from 'jotai';
+import { selectAtom } from 'jotai/utils';
 import {
   Avatar,
   Box,
@@ -267,7 +268,7 @@ function SpaceHeader() {
       <PageNavHeader>
         <Box alignItems="Center" grow="Yes" gap="300">
           <Box grow="Yes" alignItems="Center" gap="100">
-            <Text size="H4" truncate>
+            <Text as="h2" size="H4" truncate>
               {spaceName}
             </Text>
             {joinRules?.join_rule !== JoinRule.Public && <Icon src={Icons.Lock} size="50" />}
@@ -382,7 +383,25 @@ export function Space() {
   const spaceIdOrAlias = getCanonicalAliasOrRoomId(mx, space.roomId);
   const scrollRef = useRef<HTMLDivElement>(null);
   const mDirects = useAtomValue(mDirectAtom);
-  const roomToUnread = useAtomValue(roomToUnreadAtom);
+  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 allRooms = useAtomValue(allRoomsAtom);
   const allJoinedRooms = useMemo(() => new Set(allRooms), [allRooms]);
   const notificationPreferences = useRoomsNotificationPreferencesContext();
@@ -414,10 +433,10 @@ export function Space() {
           return false;
         }
         const showRoomAnyway =
-          roomToUnread.has(roomId) || roomId === selectedRoomId || callEmbed?.roomId === roomId;
+          roomsWithUnreadSet.has(roomId) || roomId === selectedRoomId || callEmbed?.roomId === roomId;
         return !showRoomAnyway;
       },
-      [space.roomId, closedCategories, roomToUnread, selectedRoomId, callEmbed]
+      [space.roomId, closedCategories, roomsWithUnreadSet, selectedRoomId, callEmbed]
     ),
     useCallback(
       (sId) => closedCategories.has(makeNavCategoryId(space.roomId, sId)),