Add FIR, BWE bitrate, and quality-limitation to TrackStats

[lkang-nuro]

Summary

Extends RTCSender.GetTrackStats so downstream RealtimeAnalytics metrics have real sources, without changing audio behavior.

• FIRCount — copied from the Pion stats interceptor's OutboundRTPStreamStats (feeds ra_webrtc_fir_count).
• AvailableOutgoingBitrate — latest GCC estimate (connection-level), stored by the Start loop. TargetBitrate — the per-track allocated share pushed to the encoder, stored in updateBitrate (feed ra_webrtc_available_outgoing_bitrate_bps / ra_webrtc_target_bitrate_bps).
• QualityLimitation{CPU,Bandwidth}Seconds — synthesized per ≈100ms tick in accumulateQualityLimitation, since pion provides no encoder feedback (feed ra_video_encoder_quality_limitation_{cpu,bandwidth}_seconds):
  • bandwidth-limited when the GCC estimate sits below the configured max bitrate;
  • CPU-limited when the encode loop falls behind (buffer eviction, tracked via encodeOverruns);
  • when GCC has no max cap, bandwidth-limited cannot be inferred and stays zero.

Out of scope

Audio send-side stats (ra_audio_*) are deferred. Audio tracks remain published externally via LiveKit and are not added to the sender's PeerConnection, so audio behavior is unchanged from main.

Testing

• go build ./..., go vet ./...
• go test ./sender/... (incl. new BitrateFields, QualityLimitation cpu/bandwidth, and FIRCount assertions)
• go test -race -run 'GetTrackStats|AddAudioTrack|SetupPeerConnection|QualityLimitation' ./sender/...
• golangci-lint run ./sender/... — 0 issues

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.63%. Comparing base (39c6229) to head (b878ed8).

Files with missing lines	Patch %	Lines
sender/rtc_sender.go	89.65%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
+ Coverage   46.03%   46.63%   +0.60%     
==========================================
  Files          18       18              
  Lines        1803     1829      +26     
==========================================
+ Hits          830      853      +23     
- Misses        905      907       +2     
- Partials       68       69       +1     

Flag	Coverage Δ
go	46.63% <89.65%> (+0.60%)	⬆️

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