Pre-encoded video publish path; room EOS emitted on disconnect

Features

• Add a pre-encoded video publish path: a passthrough video encoder and encoded video frame buffer in webrtc-sys, and EncodedVideoFrame/EncodedVideoCodec/EncodedFrameType publish APIs with a VideoEncoderBackend::PreEncoded backend in libwebrtc. WebRTC rate-control targets and keyframe requests are forwarded to encoded sources, and pre-encoded AV1 and H265 access units are validated on ingest.

Fixes

• Emit room EOS when the underlying LiveKit room event channel closes after a server-initiated disconnect, and ignore duplicate disconnect events during teardown.
• Don't log an expected publisher data channel close as unexpected - #1224 (@longcw)

Simplify x-google-start-bitrate logic and update degradation preference defaults

• Start bitrate: use min(90% of target, 1 Mbps) instead of adaptive network hints
• Remove slow connection detection and network quality hints on reconnect
• Default degradation preference by track source:
  • Camera: MaintainFramerate (smoother video)
  • Screenshare: MaintainResolution (clarity for text/UI)
  • Other: Balanced