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/types/matrix/room.ts

@@ -29,6 +29,8 @@ export enum StateEvent {
   RoomPinnedEvents = 'm.room.pinned_events',
   RoomEncryption = 'm.room.encryption',
   RoomHistoryVisibility = 'm.room.history_visibility',
+  // [MSC1763] Per-room message retention policy (disappearing messages).
+  RoomRetention = 'm.room.retention',
   RoomGuestAccess = 'm.room.guest_access',
   RoomServerAcl = 'm.room.server_acl',
   RoomTombstone = 'm.room.tombstone',