feat(audio-rooms): consume moq-lite catalog metadata in VM

The catalog subscription API shipped two commits ago but nothing
read it. This wires it up:

  * RoomSpeakerCatalog data class (commons) — kotlinx.serialization
    parser for moq-lite's `catalog.json` payload (version, audio
    track list with codec / sample_rate / channel_count / bitrate).
    Forward-compat: ignoreUnknownKeys + nullable fields tolerate
    new keys and partial publishers.
  * AudioRoomViewModel.speakerCatalogs: StateFlow<Map<String,
    RoomSpeakerCatalog>> populated lazily as per-speaker
    subscriptions land. Catalog fetch piggybacks on openSubscription
    via subscribeCatalog — best-effort, doesn't gate audio.
  * Participant context sheet renders a single-line summary
    ("OPUS · 48kHz · 2ch") below the pubkey when the catalog is
    available.
  * Enabled kotlinx.serialization plugin on :commons (was already
    a transitive dep, just not wired for @Serializable codegen).

Tests:
  * RoomSpeakerCatalogTest — 7 cases covering the canonical shape,
    describe() formatting (codec uppercased, kHz / channel
    short-formed), all-null fallback, unknown-key tolerance,
    garbage-bytes fallback, and empty-audio-list.

commons/build.gradle.kts

@@ -6,6 +6,7 @@ plugins {
     alias(libs.plugins.androidKotlinMultiplatformLibrary)
     alias(libs.plugins.jetbrainsComposeCompiler)
     alias(libs.plugins.composeMultiplatform)
+    alias(libs.plugins.serialization)
 }
 
 kotlin {