feat(audio-rooms): chat ledger on AudioRoomViewModel

Adds the listener-side state for the live-chat panel (T1 #2):

  AudioRoomViewModel.chat: StateFlow<List<LiveActivitiesChatMessageEvent>>
  AudioRoomViewModel.onChatEvent(event)

Same precedent as `presences` — the platform layer is the source
(amethyst observes LocalCache for kind=1311 + #a=[roomATag] and
pipes events here). The list is `created_at`-ascending so the chat
panel auto-scrolls newest at the bottom. Dedupes by event id so a
relay re-emit on reconnect doesn't double up the transcript.

Tests:
  * onChatEventAccumulatesMessagesSortedByCreatedAt — out-of-order
    arrivals end up in the right place on screen
  * onChatEventDedupesByEventId — same event from two relays /
    one reconnect produces ONE row

The amethyst-side wire sub + chat panel UI come next.

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

@@ -134,6 +134,21 @@ class AudioRoomViewModel(
     private val _presences = MutableStateFlow<Map<String, RoomPresence>>(emptyMap())
     val presences: StateFlow<Map<String, RoomPresence>> = _presences.asStateFlow()
 
+    /**
+     * Chat ledger for the live-activities chat panel (#1) — every
+     * kind-1311 ([com.vitorpamplona.quartz.nip53LiveActivities.chat.LiveActivitiesChatMessageEvent])
+     * tagged with this room's `a`-pointer, ordered by `created_at`
+     * ascending so the newest message is at the end (the panel
+     * auto-scrolls to it).
+     *
+     * Dedupes by event id — a relay re-emit on reconnect can't
+     * produce a duplicate row.
+     */
+    private val chatById = LinkedHashMap<String, com.vitorpamplona.quartz.nip53LiveActivities.chat.LiveActivitiesChatMessageEvent>()
+    private val _chat =
+        MutableStateFlow<List<com.vitorpamplona.quartz.nip53LiveActivities.chat.LiveActivitiesChatMessageEvent>>(emptyList())
+    val chat: StateFlow<List<com.vitorpamplona.quartz.nip53LiveActivities.chat.LiveActivitiesChatMessageEvent>> = _chat.asStateFlow()
+
     private var listener: NestsListener? = null
     private var connectJob: Job? = null
     private var stateObserverJob: Job? = null
@@ -251,6 +266,19 @@ class AudioRoomViewModel(
         _presences.value = presenceAgg.evictOlderThan(olderThanSec)
     }
 
+    /**
+     * Apply one kind-1311 chat event to the room ledger. The platform
+     * layer is the source — it filters by `a`-tag matching the current
+     * room before invoking this. Same-id re-emits are deduped; the
+     * resulting list is sorted by `created_at` ascending.
+     */
+    fun onChatEvent(event: com.vitorpamplona.quartz.nip53LiveActivities.chat.LiveActivitiesChatMessageEvent) {
+        if (closed) return
+        if (chatById.containsKey(event.id)) return
+        chatById[event.id] = event
+        _chat.value = chatById.values.sortedBy { it.createdAt }
+    }
+
     /**
      * Whether this VM was constructed with capture + encoder factories. The
      * UI uses this to decide whether to render the talk button at all —