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/config.rs

@@ -105,7 +105,7 @@ pub enum AudioProfile {
     #[default]
     Balanced,
     /// Maximum resilience on a lossy/congested link: in-band FEC tuned for heavy
-    /// loss plus DTX, at a lower bitrate to leave headroom for the redundancy.
+    /// loss, at a lower bitrate to leave headroom for the redundancy.
     BadNetwork,
 }