feat(rooms): Disappearing Messages (MSC1763 m.room.retention)

B1 of the Matrix protocol-gaps roadmap, gate-green (688 tests):
- StateEvent.RoomRetention + a shared utils/retention.ts (presets, isExpired,
  getRoomRetentionMs) with tests.
- RoomRetention settings control (PL-gated preset buttons Off/1d/1w/1m) in Room
  Settings → General → Message Retention.
- Timeline hides events past the room's max_lifetime (gated behind Show Hidden
  Events, like redactions) — messages visually disappear, losslessly.
- Opt-in setting enforceRetentionLocally (default OFF) + a headless
  RetentionSweeper that permanently redacts the user's OWN expired messages
  (own-only, loaded-timeline scope, dedupe + retry). Nothing auto-deletes unless
  the user opts in.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/RoomTimeline.tsx

@@ -109,6 +109,8 @@ import { roomIdToActiveThreadIdAtomFamily } from '../../state/room/thread';
 import { ThreadSummary } from './thread/ThreadSummary';
 import { usePowerLevelsContext } from '../../hooks/usePowerLevels';
 import { GetContentCallback, MessageEvent, StateEvent } from '../../../types/matrix/room';
+import { useStateEvent } from '../../hooks/useStateEvent';
+import { RetentionContent, isExpired } from '../../utils/retention';
 import { useKeyDown } from '../../hooks/useKeyDown';
 import { useDocumentFocusChange } from '../../hooks/useDocumentFocusChange';
 import { RenderMessageContent } from '../../components/RenderMessageContent';
@@ -468,6 +470,11 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   const [encUrlPreview] = useSetting(settingsAtom, 'encUrlPreview');
   const showUrlPreview = room.hasEncryptionStateEvent() ? encUrlPreview : urlPreview;
   const [showHiddenEvents] = useSetting(settingsAtom, 'showHiddenEvents');
+  // MSC1763 retention: messages older than this window are hidden from the
+  // timeline (unless "show hidden events" is on). Reactive so a policy change
+  // re-renders. `undefined` = no policy.
+  const retentionEvent = useStateEvent(room, StateEvent.RoomRetention);
+  const retentionMs = retentionEvent?.getContent<RetentionContent>().max_lifetime;
   const [showDeveloperTools] = useSetting(settingsAtom, 'developerTools');
   const [lotusTerminal] = useSetting(settingsAtom, 'lotusTerminal');
 
@@ -2043,6 +2050,11 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
     if (eventSender && ignoredUsersSet.has(eventSender)) {
       return null;
     }
+    // MSC1763: hide messages past the room's retention window (disappearing
+    // messages). Power users can still inspect via "show hidden events".
+    if (retentionMs && !showHiddenEvents && isExpired(mEvent.getTs(), retentionMs, Date.now())) {
+      return null;
+    }
     if (mEvent.isRedacted() && !showHiddenEvents) {
       // eslint-disable-next-line @typescript-eslint/no-shadow
       const t = mEvent.getType();