Video starts at full quality; AV1 decode fixed with trailers

Features

Improve initial video quality by setting x-google-start-bitrate SDP hint for all video codecs (VP8, VP9, AV1, H264, H265) and defaulting to MaintainResolution degradation preference.

This addresses the issue where video starts blurry for several seconds before improving, by:

1. Telling WebRTC's bandwidth estimator to start at 70% of target bitrate instead of ramping up from ~300kbps
2. Preferring frame drops over resolution reduction when bandwidth is constrained

The DegradationPreference option is now exposed via FFI for Python, C++, Unity, and Node SDKs.

Add MaintainFramerateAndResolution to DegradationPreference enum to align with WebRTC M144.

• MAINTAIN_FRAMERATE_AND_RESOLUTION is now the recommended value (replaces deprecated DISABLED)
• DISABLED is deprecated but still supported for backwards compatibility
• Both values map to the same behavior: maintain framerate and resolution, dropping frames if needed

Fixes

• Fix AV1 subscriber decode when packet trailers are enabled.