W12 follow-up: consume in-band FEC, drop redundant DTX

Fixes the two P2 efficacy findings from the Codex audit of the W12
profiles feature.

FEC was enabled on the encoder but never used: the jitter buffer's
loss path did pure PLC, so the redundancy was wasted bitrate. Now the
gap path reconstructs the lost frame from the next buffered packet via
Opus in-band FEC (new `AudioDecoder::decode_fec`, libopus decode with
fec=true into a one-frame buffer), keeping that packet for its own
normal decode and falling back to PLC if FEC decode fails. This is the
documented libopus FEC pattern; receiver-side only, no wire change.

DTX was enabled on BadNetwork but provided no benefit — the capture
noise gate already suppresses silence transmission, and the broadcast
DTX silence packets only created seq gaps that grew the jitter cushion.
All profiles now set dtx=false (plumbing kept for a future revisit).

Adds a jitter-buffer test proving FEC reconstruction beats pure PLC
(RMS error < 0.75x) and that the FEC source packet stays buffered.
500 lib tests, clippy + fmt clean, release build clean.

Co-Authored-By: Codex (gpt-5.5) <noreply@openai.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/codec/mod.rs

@@ -20,6 +20,9 @@ pub trait AudioDecoder: Send {
     /// If `compressed` is `None` (or `Some(&[])`), it indicates packet loss,
     /// enabling the decoder to perform packet loss concealment (PLC).
     fn decode(&mut self, compressed: Option<&[u8]>) -> Result<Vec<i16>, CodecError>;
+
+    /// Reconstructs the previous lost frame from the next packet's in-band FEC.
+    fn decode_fec(&mut self, next_payload: &[u8]) -> Result<Vec<i16>, CodecError>;
 }
 
 pub mod opus_impl;

src/codec/opus_impl.rs

@@ -40,7 +40,7 @@ pub fn opus_params(profile: AudioProfile) -> OpusParams {
             bitrate: 20_000,
             inband_fec: true,
             packet_loss_perc: 25,
-            dtx: true,
+            dtx: false,
         },
     }
 }
@@ -162,6 +162,19 @@ impl AudioDecoder for OpusDecoder {
         pcm.truncate(decoded_per_channel * channels_count);
         Ok(pcm)
     }
+
+    fn decode_fec(&mut self, next_payload: &[u8]) -> Result<Vec<i16>, CodecError> {
+        let channels_count = self.channels_count();
+        let mut pcm = vec![0i16; self.frame_samples * channels_count];
+
+        let decoded_per_channel = self
+            .decoder
+            .decode(next_payload, &mut pcm, true)
+            .map_err(|e| CodecError::Decode(format!("Opus FEC decoding failed: {}", e)))?;
+
+        pcm.truncate(decoded_per_channel * channels_count);
+        Ok(pcm)
+    }
 }
 
 #[cfg(test)]
@@ -180,10 +193,8 @@ mod tests {
         assert!(bal.inband_fec);
         assert!(bad.inband_fec);
 
-        // BadNetwork is the only profile that enables DTX, and it expects the
-        // heaviest loss.
-        assert!(bad.dtx);
-        assert!(!low.dtx && !bal.dtx);
+        // Capture-side gating suppresses silence; no profile adds Opus DTX.
+        assert!(!low.dtx && !bal.dtx && !bad.dtx);
         assert!(bad.packet_loss_perc > bal.packet_loss_perc);
 
         // BadNetwork trims base bitrate to make room for FEC redundancy.