- Add cache *keyFrameCache field to Source (nil by default).
- Add EnableKeyFrameCache() — call before Start() on video sources to
activate IDR burst caching.
- readLoop: call cache.push(pkt) after each successful unmarshal, before
the subscriber fanout. No lock held at push time — push acquires its
own mutex internally.
- Subscribe: snapshot the cache outside s.mu to avoid any cross-lock
complexity, then pre-fill the new channel with the burst before
registering it in the subscriber set. Uses a labeled break to stop
pre-filling if the channel is full (bufDepth too small for the burst;
the subscriber will wait for the next live keyframe instead).
ZGaetano
020a1800ce