feat(webrtc): wire keyframe cache into Source (issue #17)

- 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).

core/webrtc/source.go

@@ -19,6 +19,11 @@ type Source struct {
 	started     bool
 	closed      bool
 	done        chan struct{}
+
+	// cache is non-nil only for video sources that have had
+	// EnableKeyFrameCache() called. It holds the most recent H.264 IDR
+	// burst so new subscribers can receive a keyframe immediately.
+	cache *keyFrameCache
 }
 
 // NewSource binds a UDP socket on 127.0.0.1:port. Pass port=0 to let the OS
@@ -61,13 +66,58 @@ func (s *Source) LocalAddr() *net.UDPAddr {
 	return s.conn.LocalAddr().(*net.UDPAddr)
 }
 
+// EnableKeyFrameCache activates H.264 IDR keyframe burst caching for
+// this source. Once enabled, new calls to Subscribe() will pre-fill the
+// returned channel with the most recent IDR burst before registering it
+// in the live fanout, cutting first-frame latency for late-joining peers
+// from up to one keyframe interval to nearly zero.
+//
+// Call this on video sources only; calling it on audio sources is
+// harmless but wastes memory accumulating non-IDR packets that will
+// never trigger a cache reset.
+//
+// Must be called before Start(). Subsequent calls are no-ops.
+func (s *Source) EnableKeyFrameCache() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.cache == nil {
+		s.cache = newKeyFrameCache()
+	}
+}
+
 // Subscribe returns a new buffered channel that receives every RTP packet
 // read from the UDP socket. bufDepth is the channel buffer size; when full,
 // packets are dropped (preventing a slow subscriber from back-pressuring
 // the reader).
+//
+// If a keyframe cache is active (EnableKeyFrameCache was called), the
+// channel is pre-filled with the most recent IDR burst before being
+// registered in the live fanout, so the subscriber receives a complete
+// reference frame immediately rather than waiting for the next keyframe.
 func (s *Source) Subscribe(bufDepth int) chan *rtp.Packet {
 	ch := make(chan *rtp.Packet, bufDepth)
+
+	// Snapshot outside s.mu to avoid any cross-lock ordering issue:
+	// readLoop acquires cache.mu (in push) then s.mu (in fanout), so
+	// we must not hold s.mu while calling snapshot (which acquires
+	// cache.mu). s.cache itself is immutable after EnableKeyFrameCache.
+	var burst []*rtp.Packet
+	if s.cache != nil {
+		burst = s.cache.snapshot()
+	}
+
 	s.mu.Lock()
+	// Pre-fill with the IDR burst. Use a labeled break so that a full
+	// channel (bufDepth smaller than burst length) stops pre-filling
+	// gracefully — the subscriber will catch the next live keyframe.
+prefill:
+	for _, pkt := range burst {
+		select {
+		case ch <- pkt:
+		default:
+			break prefill
+		}
+	}
 	s.subscribers[ch] = struct{}{}
 	s.mu.Unlock()
 	return ch
@@ -118,6 +168,15 @@ func (s *Source) readLoop() {
 			continue
 		}
 
+		// Update the keyframe cache (video sources only; push is a
+		// no-op on audio sources because isH264IDRStart returns false
+		// for Opus payload types). Called before the fanout so that a
+		// subscriber joining concurrently gets a snapshot that includes
+		// this packet if it is an IDR start.
+		if s.cache != nil {
+			s.cache.push(pkt)
+		}
+
 		s.mu.Lock()
 		for ch := range s.subscribers {
 			select {