Add GCC-managed encoded Opus audio track

[lkang-nuro]

Context

The audio stream is effectively fixed bitrate: audio is pre-encoded outside the sender and pushed as a passive track via AddAudioTrack, so the GCC bandwidth estimator — which dynamically drives video bitrate — never touches it. When the network degrades, video scales down but audio holds its bandwidth.

This makes audio a first-class, GCC-managed encoded Opus track inside the sender, so the same control loop (updateBitrate → updateEncoderBitrate) that drives VP8 also drives Opus, sharing the GCC bitrate pool. Callers push raw PCM instead of pre-encoded Opus.

The change is additive: the legacy passive AddAudioTrack path is left untouched.

Changes

• Generalize EncodedTrack to carry audio (isAudio, audioTrack, audioSource); the existing local track and encodedReader/bitrateTracker/mimeType are shared by both kinds.
• AddEncodedAudioTrack(trackID, opus.Params) — builds the Opus codec selector, AudioBuffer source, mediadevices AudioTrack, and encoded reader; registers the track in s.tracks so it flows through GCC allocation.
• SendAudioFrame(trackID, pcm, sampleRate, channels) — pushes raw interleaved PCM.
• New sender/audio_buffer.go (AudioBuffer) — the audio counterpart to FrameBuffer: same initialized-gate non-blocking Read and drop-oldest bounded queue.
• updateEncoderBitrate gains a codec.BitRateController branch for Opus, clamped to [8000, 32000] bps.
• processEncodedFrames uses a 20 ms duration for audio and skips the VP8 keyframe sniff; ForceKeyFrame/recreateEncoder/Close guard on isAudio.
• Extracted attachTrackToPeerConnection helper (de-dupes AddVideoTrack).

Testing

go build ./..., go vet ./..., golangci-lint run ./... (0 issues), and go test ./... all pass. New tests in sender/audio_buffer_test.go cover buffer behavior, track creation/validation, a full PCM→Opus encode path, and the GCC bitrate-driven path.

Note

Downstream consumer wiring (cgo PushAudioPCM export, C++ raw-PCM push, Bazel .so re-pin) lives in a separate repo and is out of scope here. For GCC to genuinely drive audio, the encoded audio track must be AddTrack'd on the PeerConnection this sender owns so transport-cc feedback reaches the estimator.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.16151% with 49 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.84%. Comparing base (d6fa118) to head (f746caf).

Files with missing lines	Patch %	Lines
sender/rtc_sender.go	77.56%	24 Missing and 11 partials ⚠️
sender/audio_buffer.go	89.01%	8 Missing and 2 partials ⚠️
receiver/receiver.go	55.55%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
+ Coverage   48.79%   52.84%   +4.04%     
==========================================
  Files          19       21       +2     
  Lines        1916     2165     +249     
==========================================
+ Hits          935     1144     +209     
- Misses        908      937      +29     
- Partials       73       84      +11     

Flag	Coverage Δ
go	52.84% <83.16%> (+4.04%)	⬆️

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