The encoder set OPUS_SET_INBAND_FEC, but the decoder never invoked FEC --
the loss path went DRED -> PLC, so FEC redundancy was emitted (and paid for
in bitrate) yet never consumed.
Wire FEC recovery into AudioEngine::on_playback between DRED and PLC: copy
the next buffered packet once, try DRED, else (if the stream negotiated FEC)
decode(next_pkt, ..., fec=true), else PLC. Recovery priority is now
DRED -> FEC -> PLC. Add per-stream RemoteStream::fec_enabled_, captured from
OpusParams in init_recv_stream. Docs (voice.md) updated to match.
ctest --preset dev: 27/27 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Talon
7547b8e140