refactor(nests): per-stream channel count + AudioBroadcastConfig (I4 prep)

Split the previously global `AudioFormat.CHANNELS = 1` into a
`DEFAULT_CHANNELS` constant + per-call-site `channelCount` parameters
so a single broadcast can advertise stereo Opus without forcing every
mono call site to grow a new argument. Generalises the catalog factory
to `MoqLiteHangCatalog.opus48k(name, channels)` with memoised JSON
bytes per shape, threads a new `AudioBroadcastConfig(channelCount)`
through `connectNestsSpeaker` / `connectReconnectingNestsSpeaker` /
`MoqLiteNestsSpeaker`, and adds a `channelCount` parameter to
`MediaCodecOpusEncoder`. Production behaviour is unchanged for
mono callers (the new config defaults to mono); the listener side
already discovers the channel count from the catalog via
`NestViewModel.awaitAudioPipelineConfig`. No test or wire changes.

Phase 1 of `nestsClient/plans/2026-05-06-i4-stereo-cross-stack-scenario.md`.
The hang-interop test scaffolding (`HangInteropTest`, `runSpeakerToHangListen`,
Rust `hang-listen` / `hang-publish`, `JvmOpusEncoder`) doesn't exist on
this branch yet, so the I4 forward + reverse scenarios are deferred
until the parent T16 plan lands.

https://claude.ai/code/session_01EqJEADzH9yjSuoP5L9js8i

nestsClient/src/commonTest/kotlin/com/vitorpamplona/nestsclient/moq/lite/MoqLiteHangCatalogTest.kt

@@ -25,7 +25,7 @@ import kotlin.test.assertEquals
 
 class MoqLiteHangCatalogTest {
     @Test
-    fun opusMono48kEmitsCanonicalHangShape() {
+    fun opus48kMonoEmitsCanonicalHangShape() {
         // Byte-exact assertion: `:commons`'s `RoomSpeakerCatalogTest`
         // round-trips this same string against the parser. If either
         // side drifts from the kixelated/hang wire shape, both tests
@@ -35,17 +35,49 @@ class MoqLiteHangCatalogTest {
                 "\"codec\":\"opus\",\"container\":{\"kind\":\"legacy\"}," +
                 "\"sampleRate\":48000,\"numberOfChannels\":1,\"jitter\":20}}}}"
         val actual =
-            MoqLiteHangCatalog.opusMono48k("audio/data").encodeJsonBytes().decodeToString()
+            MoqLiteHangCatalog.opus48k("audio/data").encodeJsonBytes().decodeToString()
+        assertEquals(expected, actual)
+    }
+
+    @Test
+    fun opus48kStereoEmitsTwoChannelHangShape() {
+        // Stereo broadcasts (kixelated/moq web publisher with a stereo
+        // AudioContext) declare numberOfChannels = 2. Listeners pick
+        // this up via the catalog and configure their decoder + sink
+        // for L/R interleaved PCM.
+        val expected =
+            "{\"audio\":{\"renditions\":{\"audio/data\":{" +
+                "\"codec\":\"opus\",\"container\":{\"kind\":\"legacy\"}," +
+                "\"sampleRate\":48000,\"numberOfChannels\":2,\"jitter\":20}}}}"
+        val actual =
+            MoqLiteHangCatalog
+                .opus48k("audio/data", numberOfChannels = 2)
+                .encodeJsonBytes()
+                .decodeToString()
         assertEquals(expected, actual)
     }
 
     @Test
     fun renditionKeyMatchesCallerSuppliedTrackName() {
         val actual =
-            MoqLiteHangCatalog.opusMono48k("custom/track").encodeJsonBytes().decodeToString()
+            MoqLiteHangCatalog.opus48k("custom/track").encodeJsonBytes().decodeToString()
         // The rendition map MUST be keyed on the caller-supplied track
         // name — the watcher uses this string verbatim as the
         // SUBSCRIBE.track on the audio subscription.
         assertEquals(true, actual.contains("\"custom/track\":{"))
     }
+
+    @Test
+    fun opus48kJsonBytesMemoisesPerShape() {
+        // Same shape → same byte array reference (fast-path on every
+        // subsequent broadcast / hot-swap iteration).
+        val a = MoqLiteHangCatalog.opus48kJsonBytes("audio/data", 1)
+        val b = MoqLiteHangCatalog.opus48kJsonBytes("audio/data", 1)
+        assertEquals(true, a === b)
+
+        // Different shape → different bytes; both stay cached.
+        val stereo = MoqLiteHangCatalog.opus48kJsonBytes("audio/data", 2)
+        assertEquals(true, a !== stereo)
+        assertEquals(true, stereo === MoqLiteHangCatalog.opus48kJsonBytes("audio/data", 2))
+    }
 }