Fix initial video quality blurriness for all codecs

[xianshijing-lk]

Summary

Improve initial video quality by:

1. Capping x-google-start-bitrate at 1 Mbps to prevent BWE from starting too aggressively on high bitrate tracks
2. Setting source-based degradationPreference defaults for optimal quality per use case

Changes

PCTransport.ts

• Added 1 Mbps cap on x-google-start-bitrate: min(target * 0.9, 1000 kbps)
• Prevents bandwidth estimator from overwhelming the network on high bitrate tracks (e.g., 4K video)

publishUtils.ts

• Updated getDefaultDegradationPreference() to return defaults based on track source:
  • Camera: maintain-framerate (smoother video for real-time communication)
  • Screen share: maintain-resolution (clarity is critical for text/UI)
  • Other/unknown: balanced

options.ts

• Updated documentation for degradationPreference option

Behavior
│ Setting │ Before │ After │
│ x-google-start-bitrate │ 90% of target (unbounded) │ min(90% of target, 1 Mbps) │
│ Camera degradationPreference │ maintain-resolution │ maintain-framerate │
│ Screen share degradationPreference │ maintain-resolution │ maintain-resolution │
│ Other degradationPreference │ maintain-resolution │ balanced │

Related

Aligns with rust-sdks PRs:

• await element to be attached before calling .play on it #1197 - Fix initial video quality blurriness for all codecs
• Ensure SVC layers are starting from LOW quality #1226 - fix x-google-start-bitrate and make it adaptive to network

See Rust PR: livekit/rust-sdks#1197

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size-limit report 📦

Path	Size
dist/livekit-client.esm.mjs	101.45 KB (-0.03% 🔽)
dist/livekit-client.umd.js	110.52 KB (+0.15% 🔺)