feat: RoomUpdate protocol — broadcast participant list on join/leave

- Add RoomUpdate signal message to wzp-proto with participant count + list
- Add RoomParticipant struct (fingerprint + optional alias)
- Store fingerprint/alias in relay Participant struct
- Broadcast RoomUpdate to all room members on join and leave
- Add signal recv task in Android engine to handle RoomUpdate
- Surface room_participant_count + room_participants in CallStats JSON
- Show "X in room" with participant names in Android in-call UI

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-android/src/stats.rs

@@ -53,4 +53,15 @@ pub struct CallStats {
     pub fec_recovered: u64,
     /// Current mic audio level (RMS of i16 samples, 0-32767).
     pub audio_level: u32,
+    /// Number of participants in the room (from last RoomUpdate).
+    pub room_participant_count: u32,
+    /// Participant list (fingerprint + optional alias) serialized as JSON array.
+    pub room_participants: Vec<RoomMember>,
+}
+
+/// A room member entry, serialized into the stats JSON.
+#[derive(Clone, Debug, Default, serde::Serialize)]
+pub struct RoomMember {
+    pub fingerprint: String,
+    pub alias: Option<String>,
 }