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>

android/app/src/main/java/com/wzp/engine/CallStats.kt

@@ -1,5 +1,6 @@
 package com.wzp.engine
 
+import org.json.JSONArray
 import org.json.JSONObject
 
 /**
@@ -32,6 +33,10 @@ data class CallStats(
     val fecRecovered: Long = 0,
     /** Current mic audio level (RMS, 0-32767). */
     val audioLevel: Int = 0,
+    /** Number of participants in the room. */
+    val roomParticipantCount: Int = 0,
+    /** Participants in the room (fingerprint + optional alias). */
+    val roomParticipants: List<RoomMember> = emptyList(),
 ) {
     /** Human-readable quality label. */
     val qualityLabel: String
@@ -43,6 +48,17 @@ data class CallStats(
         }
 
     companion object {
+        private fun parseParticipants(arr: JSONArray?): List<RoomMember> {
+            if (arr == null) return emptyList()
+            return (0 until arr.length()).map { i ->
+                val o = arr.getJSONObject(i)
+                RoomMember(
+                    fingerprint = o.optString("fingerprint", ""),
+                    alias = o.optString("alias", null)
+                )
+            }
+        }
+
         /** Deserialise from the JSON string produced by the native engine. */
         fun fromJson(json: String): CallStats {
             return try {
@@ -59,7 +75,9 @@ data class CallStats(
                     framesDecoded = obj.optLong("frames_decoded", 0),
                     underruns = obj.optLong("underruns", 0),
                     fecRecovered = obj.optLong("fec_recovered", 0),
-                    audioLevel = obj.optInt("audio_level", 0)
+                    audioLevel = obj.optInt("audio_level", 0),
+                    roomParticipantCount = obj.optInt("room_participant_count", 0),
+                    roomParticipants = parseParticipants(obj.optJSONArray("room_participants"))
                 )
             } catch (e: Exception) {
                 CallStats()
@@ -67,3 +85,12 @@ data class CallStats(
         }
     }
 }
+
+data class RoomMember(
+    val fingerprint: String,
+    val alias: String? = null
+) {
+    /** Short display name: alias if set, otherwise first 8 chars of fingerprint. */
+    val displayName: String
+        get() = alias ?: fingerprint.take(8)
+}

android/app/src/main/java/com/wzp/ui/call/InCallScreen.kt

@@ -228,6 +228,22 @@ fun InCallScreen(
 
                 QualityIndicator(qualityTier, stats.qualityLabel)
 
+                if (stats.roomParticipantCount > 0) {
+                    Spacer(modifier = Modifier.height(8.dp))
+                    Text(
+                        text = "${stats.roomParticipantCount} in room",
+                        style = MaterialTheme.typography.bodySmall,
+                        color = MaterialTheme.colorScheme.onSurfaceVariant
+                    )
+                    stats.roomParticipants.forEach { member ->
+                        Text(
+                            text = member.displayName,
+                            style = MaterialTheme.typography.labelSmall,
+                            color = MaterialTheme.colorScheme.onSurfaceVariant
+                        )
+                    }
+                }
+
                 Spacer(modifier = Modifier.height(32.dp))
 
                 AudioLevelBar(stats.audioLevel)