feat(audio-rooms): announce-driven speaker discovery (T4 #17b)

Surface moq-lite's ANNOUNCE flow on NestsListener so the audio
room can render an authoritative "actively broadcasting"
indicator independent of kind-10312 presence's `publishing`
flag. Same channel nostrnests' web client uses for its live
badges (`useRoomAnnouncements` in the JS reference).

Wire-up:
  * RoomAnnouncement(pubkey, active) data class — one update
    per publisher transition (Active → broadcast came up,
    inactive → broadcast went down).
  * NestsListener.announces(): Flow<RoomAnnouncement> with a
    default body that throws UnsupportedOperationException on
    the IETF reference path.
  * MoqLiteNestsListener implements via session.announce("")
    against the room's namespace; the suffix carries the
    speaker pubkey hex straight through.
  * ReconnectingNestsListener routes via collectLatest so the
    consumer-facing flow restarts against each new session
    after a refresh / reconnect (no SubscribeHandle re-issuance
    pump needed — announces is a cold per-collect stream).
  * AudioRoomViewModel.announcedSpeakers: StateFlow<Set<String>>
    populated by observeAnnounces. Active emissions add the
    pubkey, inactive emissions remove it. Cleared on teardown.
    IETF listeners leave the set empty; UI falls back to
    presence's publishingNow flag.

Tests:
  * NestsListenerCatalogTest — adds the announces() default-
    throws-on-collect case.

Closes the audit's Tier-4 #17b gap. UI integration (e.g. a green
"live" dot driven by announcedSpeakers) is a follow-up — the data
flow is in place and downstream consumers can opt in.

nestsClient/src/commonTest/kotlin/com/vitorpamplona/nestsclient/NestsListenerCatalogTest.kt

@@ -53,4 +53,19 @@ class NestsListenerCatalogTest {
                 listener.subscribeCatalog("speakerPubkey")
             }
         }
+
+    @Test
+    fun ietfDefaultListenerAnnouncesFlowThrowsOnCollect() =
+        runTest {
+            // The announce-prefix channel is moq-lite only as well —
+            // the IETF reference path's announces() default body
+            // throws UnsupportedOperationException at collect time.
+            // Callers mixing the two protocols can `runCatching` the
+            // collect to no-op cleanly.
+            val listener = IetfStyleListener()
+            val flow = listener.announces()
+            assertFailsWith<UnsupportedOperationException> {
+                flow.collect { /* unreachable */ }
+            }
+        }
 }