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/commonTest/kotlin/com/vitorpamplona/amethyst/commons/viewmodels/AudioRoomViewModelTest.kt

@@ -151,6 +151,69 @@ class AudioRoomViewModelTest {
             assertTrue(vm.uiState.value.onStageNow)
         }
 
+    @Test
+    fun onPresenceEventPopulatesPresencesMapAndDedupesByPubkey() =
+        runTest {
+            val vm = newViewModel { FakeNestsListener() }
+
+            val alice = "a".repeat(64)
+            val bob = "b".repeat(64)
+
+            fun ev(
+                pk: String,
+                createdAt: Long,
+                handRaised: Boolean,
+            ) = com.vitorpamplona.quartz.nip53LiveActivities.presence.MeetingRoomPresenceEvent(
+                id = "0".repeat(64),
+                pubKey = pk,
+                createdAt = createdAt,
+                tags = arrayOf(arrayOf("a", "30312:host:room"), arrayOf("hand", if (handRaised) "1" else "0")),
+                content = "",
+                sig = "0".repeat(128),
+            )
+
+            vm.onPresenceEvent(ev(alice, 100L, handRaised = false))
+            vm.onPresenceEvent(ev(bob, 100L, handRaised = false))
+            vm.onPresenceEvent(ev(alice, 200L, handRaised = true))
+
+            val snap = vm.presences.value
+            assertEquals(setOf(alice, bob), snap.keys)
+            assertTrue(snap[alice]!!.handRaised)
+            assertEquals(200L, snap[alice]!!.updatedAtSec)
+        }
+
+    @Test
+    fun evictStalePresencesDropsOldPeers() =
+        runTest {
+            val vm = newViewModel { FakeNestsListener() }
+            val alice = "a".repeat(64)
+            val bob = "b".repeat(64)
+
+            vm.onPresenceEvent(
+                com.vitorpamplona.quartz.nip53LiveActivities.presence.MeetingRoomPresenceEvent(
+                    id = "0".repeat(64),
+                    pubKey = alice,
+                    createdAt = 100L,
+                    tags = arrayOf(arrayOf("a", "30312:host:room")),
+                    content = "",
+                    sig = "0".repeat(128),
+                ),
+            )
+            vm.onPresenceEvent(
+                com.vitorpamplona.quartz.nip53LiveActivities.presence.MeetingRoomPresenceEvent(
+                    id = "0".repeat(64),
+                    pubKey = bob,
+                    createdAt = 500L,
+                    tags = arrayOf(arrayOf("a", "30312:host:room")),
+                    content = "",
+                    sig = "0".repeat(128),
+                ),
+            )
+
+            vm.evictStalePresences(olderThanSec = 400L)
+            assertEquals(setOf(bob), vm.presences.value.keys)
+        }
+
     @Test
     fun publishingNowDerivesFromBroadcastStateAndMute() {
         // Idle / connecting / failed: never publishing.

commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/viewmodels/RoomPresenceStateTest.kt

@@ -116,12 +116,14 @@ class RoomPresenceStateTest {
     }
 
     @Test
-    fun roomPresenceEqualityIsPubkeyOnly() {
-        // Two snapshots of the same peer at different times are
-        // "equal" so a Set<RoomPresence> deduplicates correctly.
+    fun roomPresenceEqualityIsAllFields() {
+        // Two snapshots of the same peer at different timestamps are
+        // NOT equal — Map<String, RoomPresence>.equals(Map) reaches
+        // value.equals(value); a pubkey-only equals would suppress
+        // StateFlow emissions on heartbeat-only updates because
+        // old.equals(new) would return true.
         val a1 = RoomPresence.from(event(alice, 100L, handRaised = false))
         val a2 = RoomPresence.from(event(alice, 200L, handRaised = true))
-        assertEquals(a1, a2)
-        assertEquals(a1.hashCode(), a2.hashCode())
+        assertEquals(false, a1 == a2)
     }
 }