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>

LOTUS_TESTING.md

@@ -675,6 +675,8 @@ Run the axe DevTools extension (or Lighthouse → Accessibility) on a room view,
 
 ## Outstanding verification backlog
 
+**Disappearing Messages (MSC1763 `m.room.retention`, 2026-07):** In Room Settings → General → **Message Retention**, an admin picks Off / 1 Day / 1 Week / 1 Month (non-admins see the buttons disabled). After setting e.g. 1 Day, messages older than a day **vanish from the timeline** for everyone in Lotus (toggle Settings → General → **Show Hidden Events** to reveal them again). Setting back to **Off** restores them. Separately, each user can enable Settings → General → **Enforce Message Retention** (default OFF) → their OWN expired messages then get **permanently redacted** within ~30 s (verify: OTHER people's messages are NEVER redacted by this; only your own). Note true server-side purge also needs Synapse `retention:` configured.
+
 **Mark as Unread + Low Priority (MSC2867 / m.lowpriority, 2026-07):** Right-click a room in the sidebar → **Mark as Unread** puts a dot on the row (bold name) even with no new messages; opening/reading the room clears it, and it syncs to another device. **Mark as Read** on a marked room clears it too. Right-click → **Add to Low Priority** moves the room into a collapsed "Low Priority" category at the bottom of the room list (and removes it from Favorites if it was there, and vice-versa); **Remove from Low Priority** returns it to Rooms.
 
 **Windows rich toast (D6, 2026-07 — desktop/Windows build only):** get a message notification while the desktop app is backgrounded → the toast is attributed to **Lotus Chat** (not "PowerShell"/generic) and shows an inline **reply box + Send**; typing a reply + Send **posts it to that room**; clicking the toast body **opens the room**. Previously these silently fell back to a plain toast (no reply/click). If it still falls back, check that a `Lotus Chat.lnk` exists in the Start-Menu Programs folder.