fix(audio): wire in-band FEC into the decoder loss path

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>

PROGRESS.md

@@ -691,6 +691,14 @@ iOS 18.0 deployment target. App Group `group.cat.voice.VoiceCat` for Keychain sh
   reconstructs the lost frame — otherwise falls back to standard PLC. New test:
   `test_dred_toggle` (ctest 22/22). Files: `voicecat.proto`, `voicecat.h`,
   `opus_codec.{h,cpp}`, `audio_engine.{h,cpp}`, `client.cpp`, `session.{h,cpp}`.
+- [x] **In-band FEC decoder wiring** — done (2026-06-22). The encoder set `OPUS_SET_INBAND_FEC`
+  all along, but the decoder never invoked it — the loss path went DRED → PLC, so FEC redundancy
+  was emitted (and paid for in bitrate) but never consumed. Wired the FEC recovery into
+  `AudioEngine::on_playback`'s loss branch 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.
+  Added per-stream `RemoteStream::fec_enabled_`, captured from `OpusParams` in
+  `init_recv_stream`. Recovery priority is now **DRED → FEC → PLC**. ctest 27/27 green. Files:
+  `audio_engine.{h,cpp}`, `docs/voice.md`.
 - [ ] **DRED toggle in client UIs** — expose the `dred` flag in all three channel-config UIs
   so admins can enable it per channel. Windows: `ChannelEditForm` / `vc_channel_info.audio.dred`
   checkbox. macOS AppKit: channel-edit sheet. iOS SwiftUI: channel-edit form. All three UIs