feat: voice channel user limit (P5-10) + call join/leave sounds (P5-16)

P5-10 Voice Channel User Limit:
- New StateEvent.LotusVoiceLimit (io.lotus.voice_limit) with { max_users }
- RoomVoiceLimit admin control in Room Settings > General > Voice
  (power-level gated via permissions.stateEvent)
- CallPrescreen reads the limit reactively and disables Join with a
  'Channel Full (N/N)' message at capacity; existing members can rejoin

P5-16 Custom Join/Leave Sound Effects:
- useCallJoinLeaveSounds hook wired into CallUtils; detects participant
  join/leave via MatrixRTCSession membership changes (sender|deviceId),
  filters out self, only fires while joined
- Sounds synthesized in-browser with Web Audio (callSounds.ts) - no
  assets bundled; styles Off/Chime/Soft/Retro
- 'Join & Leave Sounds' selector in Settings > Calls (previews on change)

Docs: LOTUS_FEATURES.md, README.md, LOTUS_TODO.md (P5-10/P5-16 marked done)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

LOTUS_TODO.md

@@ -256,11 +256,12 @@ Themes:
 
 ---
 
-### [ ] P5-10 · Voice Channel User Limit
+### [x] P5-10 · Voice Channel User Limit
 
 **What:** Admins set max participants via custom state event `io.lotus.voice_limit: { max_users: N }`. Show "Channel Full (5/5)" to users over the limit. Local enforcement only.  
 **[AUDIT REQUIRED]** Check if Element Call has its own participant limit that should be integrated with rather than duplicated.  
-**Complexity:** Medium.
+**Complexity:** Medium.  
+**Done:** `RoomVoiceLimit` admin control in Room Settings → General → Voice; `CallPrescreen` disables Join + shows "Channel Full (N/N)" when at capacity (rejoiners exempt). State event `StateEvent.LotusVoiceLimit`.
 
 ---
 
@@ -310,11 +311,12 @@ Themes:
 
 ---
 
-### [ ] P5-16 · Custom Join / Leave Sound Effects
+### [x] P5-16 · Custom Join / Leave Sound Effects
 
 **What:** Local-only sounds when participants join/leave a call you're in. Built-in options + per-user settable. Detect via Element Call participant list change events.  
 **[AUDIT REQUIRED]** Find how Element Call exposes join/leave participant events to the parent window via postMessage bridge.  
-**Complexity:** Medium.
+**Complexity:** Medium.  
+**Done:** Detected via `MatrixRTCSession` membership changes (`useCallMembersChange`) rather than the EC postMessage bridge — more reliable, identity tracked by `sender|deviceId`. Sounds synthesized with Web Audio (no assets). Styles Off/Chime/Soft/Retro in Settings → Calls. Hook `useCallJoinLeaveSounds`, util `callSounds.ts`.
 
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