Fix WebRTC alignment and RN volume on LiveKit voice sessions

[nathanpogue6]

Fixes WebRTC/LiveKit voice sessions dropping audio_event.alignment on the data channel (playback correctly uses LiveKit tracks, but timing data is needed for karaoke highlighting). Alignment-only audio events are now forwarded to onAudioAlignment without playback payload. Stale alignment after barge-in is ignored using the same interrupt event_id guard as onAudio. React Native setVolume now applies to LiveKit remote audio tracks when no web audio adapter is present. WebSocket voice is unchanged; tests cover LiveKit, WebSocket, and interrupt behavior.

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Note

Medium Risk
Touches real-time voice event handling and WebRTC data-channel filtering; behavior is well covered by new tests but affects karaoke/alignment UX and RN volume on LiveKit sessions.

Overview
Fixes LiveKit/WebRTC voice sessions dropping character alignment on the data channel while playback still uses remote audio tracks. WebRTCConnection now forwards alignment-only audio events (empty audio_base_64) and ignores audio data messages without alignment.

VoiceConversation.handleAudio applies the same interrupt event_id guard to onAudioAlignment as to playback callbacks, so stale alignment after barge-in is ignored. Alignment-only events no longer trigger onAudio, mode changes, or feedback updates.

On platforms without a web audio adapter (e.g. React Native), output volume is applied via LiveKit RemoteAudioTrack.setVolume, with remote tracks tracked through subscribe/unsubscribe lifecycle.

New tests cover WebRTC data channel behavior, WebSocket alignment forwarding, interrupt staleness, and LiveKit volume fallback.

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[kraenhansen]

@nathanpogue6 do you know if this is a regression or if it just never worked?