[tiredIsa]

For syrnike13/syrnike13-app#157 (screen-share audio: crackling, lag, mono, audio/video sync).

Changes

• Stereo publish option. TrackPublishOptions::stereo (FFI TrackPublishOptions.stereo = 14) adds TF_STEREO to the AddTrackRequest. The server answers the publisher and offers subscribers stereo=1. Before this, two-channel screen audio was encoded as mono Opus.
• Application playout delay. livekit::setAudioPlayoutDelay(ms) (C ABI livekit_ffi_set_audio_playout_delay) is a process-wide value that the synthetic audio device returns as PlayoutDelay. WebRTC's ChannelReceive subtracts it from the audio playout position, so audio/video sync (RtpStreamsSynchronizer) accounts for audio the application renders itself after the SDK. It is clamped to 0..1000 ms.
• Synchronous unbuffered capture. For queue_size_ms = 0 sources, AudioSource::captureFrame now sends CaptureAudioFrameRequest.synchronous = true. The FFI calls the new NativeAudioSource::capture_frame_direct in the calling thread, without a runtime task or completion callback. Previously, with every core busy, a realtime capture thread waited up to 440 ms for the tokio task and the FFI event thread, and 395 of 2,022 screen-audio packets were dropped in 20 s. Errors now surface as the call's own exception.

Validation (local Windows x64 build)

• livekit_unit_tests 377/377, including the new TrackPublishOptionsTest.StereoRoundTrip and AudioSourceTest.UnbufferedCaptureCompletesInTheCallingThread.
• cargo test -p livekit-ffi --lib conversion::room, including stereo_publish_option_round_trips. cargo fmt --check is clean.
• 0001-native-v2-runtime.patch applies with git apply --check to a fresh 1a477bc4 checkout.
• App call-bench against the project's LiveKit fork with sync_streams:
  • subscribers negotiate opus/2 stereo=1;
  • the screen-audio/video playout offset is 7–38 ms with 150 ms of injected sender skew;
  • with every core busy, the engine's screen audio has 0 drops and a maximum captureFrame of 0.12 ms (it was 440 ms).

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📥 Commits

Reviewing files that changed from the base of the PR and between 1719e9b and d5bf74e.

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📒 Files selected for processing (9)

• include/livekit/livekit.h
• include/livekit/room_event_types.h
• patches/client-sdk-rust/0001-native-v2-runtime.patch
• src/audio_source.cpp
• src/livekit.cpp
• src/room_proto_converter.cpp
• src/tests/unit/test_audio_source.cpp
• src/tests/unit/test_room_event_types.cpp
• src/tests/unit/test_video_frame_metadata.cpp

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