feat(audio-rooms): expose presences StateFlow on AudioRoomViewModel

Adds the listener-side fan-in API the participant grid + listener
counter consume:

  AudioRoomViewModel.presences: StateFlow<Map<String, RoomPresence>>
  AudioRoomViewModel.onPresenceEvent(MeetingRoomPresenceEvent)
  AudioRoomViewModel.evictStalePresences(olderThanSec: Long)

The platform layer (amethyst, next commit) observes LocalCache for
`kinds=[10312], #a=[roomATag]` and pipes events through onPresenceEvent;
that keeps the data source platform-specific and lets commons stay free
of Android-only LocalCache references. evictStalePresences runs on a
periodic tick driven by the platform layer.

Bug-fix while wiring this: the initial RoomPresence had a custom
pubkey-only equals() (intended to make Set<RoomPresence> dedup
straightforwardly). That broke Map<String, RoomPresence>.equals on a
heartbeat-only update — old.equals(new) returned true even when
handRaised / publishing flipped, so StateFlow suppressed the emission
and the UI never saw the change. Reverted to the data-class
all-fields equals; the Map is keyed by the pubkey String anyway, so
the custom equals never bought us anything. Test
`roomPresenceEqualityIsAllFields` pins the contract so it can't drift
back.

Tests:
  * onPresenceEventPopulatesPresencesMapAndDedupesByPubkey
  * evictStalePresencesDropsOldPeers

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/viewmodels/AudioRoomViewModel.kt

@@ -119,6 +119,21 @@ class AudioRoomViewModel(
     private val _uiState = MutableStateFlow(AudioRoomUiState())
     val uiState: StateFlow<AudioRoomUiState> = _uiState.asStateFlow()
 
+    /**
+     * Listener-side aggregation of every peer's most recent kind-10312
+     * presence in this room. Populated by the platform layer
+     * (amethyst observes [LocalCache] for `kinds=[10312], #a=[roomATag]`
+     * and pipes events through [onPresenceEvent]) — keeping the data
+     * source platform-specific lets commons stay free of
+     * Android-only references.
+     *
+     * The participant grid (Tier 2 #1), hand-raise queue (T1 #5), and
+     * listener counter (T1 #8) all subscribe to this StateFlow.
+     */
+    private val presenceAgg = RoomPresenceAggregator()
+    private val _presences = MutableStateFlow<Map<String, RoomPresence>>(emptyMap())
+    val presences: StateFlow<Map<String, RoomPresence>> = _presences.asStateFlow()
+
     private var listener: NestsListener? = null
     private var connectJob: Job? = null
     private var stateObserverJob: Job? = null
@@ -213,6 +228,29 @@ class AudioRoomViewModel(
         _uiState.update { it.copy(onStageNow = onStage) }
     }
 
+    /**
+     * Apply one kind-10312 presence event to the in-memory aggregator.
+     * Caller owns the LocalCache → VM plumbing (the platform layer
+     * filters events by the current room's `a`-tag before invoking
+     * this). Out-of-order arrivals can't downgrade fresher state —
+     * see [RoomPresenceAggregator.apply].
+     */
+    fun onPresenceEvent(event: com.vitorpamplona.quartz.nip53LiveActivities.presence.MeetingRoomPresenceEvent) {
+        if (closed) return
+        _presences.value = presenceAgg.apply(event)
+    }
+
+    /**
+     * Drop peers whose last heartbeat is older than [olderThanSec].
+     * The platform layer drives this on a periodic tick (typically
+     * every 60 s with `now - 6 * 60` so a peer that's missed one
+     * heartbeat plus a 5-min "still here" window gets evicted).
+     */
+    fun evictStalePresences(olderThanSec: Long) {
+        if (closed) return
+        _presences.value = presenceAgg.evictOlderThan(olderThanSec)
+    }
+
     /**
      * Whether this VM was constructed with capture + encoder factories. The
      * UI uses this to decide whether to render the talk button at all —

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/viewmodels/RoomPresenceState.kt

@@ -29,8 +29,11 @@ import com.vitorpamplona.quartz.nip53LiveActivities.presence.MeetingRoomPresence
  * the relay subscription, and the participant grid + listener
  * counter render off it.
  *
- * Equality is by pubkey alone so a `Map<String, RoomPresence>` swap
- * on update doesn't double-count when only the timestamp changes.
+ * Standard data-class equality (all fields). A pubkey-only equals
+ * was tempting but breaks `Map.equals(Map)` change detection on a
+ * heartbeat-only update — `StateFlow` would suppress the emission
+ * because old.equals(new) returns true even though
+ * handRaised / publishing flipped.
  */
 @Immutable
 data class RoomPresence(
@@ -41,10 +44,6 @@ data class RoomPresence(
     val onstage: Boolean,
     val updatedAtSec: Long,
 ) {
-    override fun equals(other: Any?): Boolean = other is RoomPresence && other.pubkey == pubkey
-
-    override fun hashCode(): Int = pubkey.hashCode()
-
     companion object {
         /**
          * Project a kind-10312 event into a [RoomPresence]. Tags missing