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

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

@@ -1074,7 +1074,7 @@ class NestViewModel(
     /**
      * Wait briefly for [pubkey]'s catalog to land in [_speakerCatalogs]
      * and pick the audio config (channel count + sample rate) for the
-     * decoder + AudioTrack. Falls back to [AudioFormat.CHANNELS] /
+     * decoder + AudioTrack. Falls back to [AudioFormat.DEFAULT_CHANNELS] /
      * [AudioFormat.SAMPLE_RATE_HZ] on timeout (the catalog never
      * arrived within [timeoutMs]) or when the catalog declares
      * unsupported values (channelCount outside `1..2`, or non-positive
@@ -1108,7 +1108,7 @@ class NestViewModel(
         val channels =
             when {
                 declaredChannels == null -> {
-                    AudioFormat.CHANNELS
+                    AudioFormat.DEFAULT_CHANNELS
                 }
 
                 declaredChannels !in 1..2 -> {
@@ -1116,7 +1116,7 @@ class NestViewModel(
                         "publisher catalog for pubkey='${pubkey.take(8)}' declares numberOfChannels=$declaredChannels " +
                             "(only 1 / 2 supported); falling back to mono"
                     }
-                    AudioFormat.CHANNELS
+                    AudioFormat.DEFAULT_CHANNELS
                 }
 
                 else -> {

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

@@ -189,7 +189,7 @@ class RoomSpeakerCatalogTest {
 
     @Test
     fun stripPrefixRoundTripsCanonicalCatalog() {
-        // The catalog payload `MoqLiteHangCatalog.opusMono48k(...)` emits
+        // The catalog payload `MoqLiteHangCatalog.opus48k(...)` emits
         // (in `:nestsClient`) MUST round-trip through this parser — the
         // two classes target the same wire shape independently because
         // `:nestsClient` does not depend on `:commons` and vice versa.