feat: WarzonePhone lossy VoIP protocol — Phase 1 complete

Rust workspace with 7 crates implementing a custom VoIP protocol
designed for extremely lossy connections (5-70% loss, 100-500kbps,
300-800ms RTT). 89 tests passing across all crates.

Crates:
- wzp-proto: Wire format, traits, adaptive quality controller, jitter buffer, session FSM
- wzp-codec: Opus encoder/decoder (audiopus), Codec2 stubs, adaptive switching, resampling
- wzp-fec: RaptorQ fountain codes, interleaving, block management (proven 30-70% loss recovery)
- wzp-crypto: X25519+ChaCha20-Poly1305, Warzone identity compatible, anti-replay, rekeying
- wzp-transport: QUIC via quinn with DATAGRAM frames, path monitoring, signaling streams
- wzp-relay: Integration stub (Phase 2)
- wzp-client: Integration stub (Phase 2)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-codec/src/opus_dec.rs

@@ -0,0 +1,93 @@
+//! Opus decoder wrapping the `audiopus` crate.
+
+use audiopus::coder::Decoder;
+use audiopus::{Channels, MutSignals, SampleRate};
+use audiopus::packet::Packet;
+use wzp_proto::{AudioDecoder, CodecError, CodecId, QualityProfile};
+
+/// Opus decoder implementing `AudioDecoder`.
+///
+/// Operates at 48 kHz mono output.
+pub struct OpusDecoder {
+    inner: Decoder,
+    codec_id: CodecId,
+    frame_duration_ms: u8,
+}
+
+// SAFETY: Same reasoning as OpusEncoder — exclusive access via &mut self.
+unsafe impl Sync for OpusDecoder {}
+
+impl OpusDecoder {
+    /// Create a new Opus decoder for the given quality profile.
+    pub fn new(profile: QualityProfile) -> Result<Self, CodecError> {
+        let decoder = Decoder::new(SampleRate::Hz48000, Channels::Mono)
+            .map_err(|e| CodecError::DecodeFailed(format!("opus decoder init: {e}")))?;
+
+        Ok(Self {
+            inner: decoder,
+            codec_id: profile.codec,
+            frame_duration_ms: profile.frame_duration_ms,
+        })
+    }
+
+    /// Expected number of output PCM samples per frame.
+    pub fn frame_samples(&self) -> usize {
+        (48_000 * self.frame_duration_ms as usize) / 1000
+    }
+}
+
+impl AudioDecoder for OpusDecoder {
+    fn decode(&mut self, encoded: &[u8], pcm: &mut [i16]) -> Result<usize, CodecError> {
+        let expected = self.frame_samples();
+        if pcm.len() < expected {
+            return Err(CodecError::DecodeFailed(format!(
+                "output buffer too small: need {expected}, got {}",
+                pcm.len()
+            )));
+        }
+        let packet = Packet::try_from(encoded)
+            .map_err(|e| CodecError::DecodeFailed(format!("invalid packet: {e}")))?;
+        let signals = MutSignals::try_from(pcm)
+            .map_err(|e| CodecError::DecodeFailed(format!("output signals: {e}")))?;
+        let n = self
+            .inner
+            .decode(Some(packet), signals, false)
+            .map_err(|e| CodecError::DecodeFailed(format!("opus decode: {e}")))?;
+        Ok(n)
+    }
+
+    fn decode_lost(&mut self, pcm: &mut [i16]) -> Result<usize, CodecError> {
+        let expected = self.frame_samples();
+        if pcm.len() < expected {
+            return Err(CodecError::DecodeFailed(format!(
+                "output buffer too small: need {expected}, got {}",
+                pcm.len()
+            )));
+        }
+        let signals = MutSignals::try_from(pcm)
+            .map_err(|e| CodecError::DecodeFailed(format!("output signals: {e}")))?;
+        let n = self
+            .inner
+            .decode(None, signals, false)
+            .map_err(|e| CodecError::DecodeFailed(format!("opus PLC: {e}")))?;
+        Ok(n)
+    }
+
+    fn codec_id(&self) -> CodecId {
+        self.codec_id
+    }
+
+    fn set_profile(&mut self, profile: QualityProfile) -> Result<(), CodecError> {
+        match profile.codec {
+            CodecId::Opus24k | CodecId::Opus16k | CodecId::Opus6k => {
+                self.codec_id = profile.codec;
+                self.frame_duration_ms = profile.frame_duration_ms;
+                Ok(())
+            }
+            other => Err(CodecError::UnsupportedTransition {
+                from: self.codec_id,
+                to: other,
+            }),
+        }
+    }
+}