Implements FlexFEC-03 (RFC 8627) end to end in the SFU, in both directions:
- Upstream: decode FlexFEC repair packets from publishers and recover lost
media in the receive buffer before forwarding (pkg/sfu/flexfec/decoder.go,
pkg/sfu/buffer). Recovered packets are surfaced via Prometheus counters.
- Downstream: generate FlexFEC for subscribers off the downtrack, paced
alongside media (pkg/sfu/downtrack_fec.go, pkg/sfu/pacer).
- Negotiation: advertise and match flexfec-03 in the media engine and
transport SDP, gated by new config knobs (pkg/rtc, pkg/config,
config-sample.yaml).
- Telemetry: livekit_fec_* metrics for sent/received/recovered packets
(pkg/telemetry/prometheus/packets.go).
Tests: unit coverage for the decoder, buffer recovery, downtrack generation,
and transport negotiation, plus an end-to-end integration test and the
test-client support it needs (test/flexfec_test.go, test/client).
David Chen
195adeb38c