When a down track is added, replay the publisher's most recent group-of-pictures
so the subscriber gets a decodable stream immediately, instead of triggering a
PLI that spikes the publisher uplink and re-sends a key frame to every subscriber.
The cache is opt-in per published track via EnableVideoCaching (with
VideoCachingMaxDuration bounding the cached GOP); when disabled the ingest and
bucket-sizing paths are byte-for-byte the original logic. Adds hit/miss and
forwarded-PLI counters for observability.
Squashed from branch duan/video-frame-cache-relay (video frame caching only;
the start-at-desired-quality work is already in master).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
cloudwebrtc