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/codec2_dec.rs

@@ -0,0 +1,67 @@
+//! Codec2 decoder — stub implementation.
+//!
+//! Codec2 operates at 8 kHz mono. Resampling back to 48 kHz is handled
+//! externally (see `resample.rs` and `AdaptiveCodec`).
+//!
+//! This is a stub that returns an error on decode. When `codec2-sys`
+//! is linked, replace the body of `decode()` with actual FFI calls.
+
+use wzp_proto::{AudioDecoder, CodecError, CodecId, QualityProfile};
+
+/// Stub Codec2 decoder implementing `AudioDecoder`.
+///
+/// Currently returns `CodecError::DecodeFailed` for decode operations.
+/// PLC fills output with silence (zeros).
+pub struct Codec2Decoder {
+    codec_id: CodecId,
+    frame_duration_ms: u8,
+}
+
+impl Codec2Decoder {
+    /// Create a new stub Codec2 decoder.
+    pub fn new(profile: QualityProfile) -> Result<Self, CodecError> {
+        Ok(Self {
+            codec_id: profile.codec,
+            frame_duration_ms: profile.frame_duration_ms,
+        })
+    }
+
+    /// Expected number of 8 kHz PCM output samples per frame.
+    pub fn frame_samples(&self) -> usize {
+        (8_000 * self.frame_duration_ms as usize) / 1000
+    }
+}
+
+impl AudioDecoder for Codec2Decoder {
+    fn decode(&mut self, _encoded: &[u8], _pcm: &mut [i16]) -> Result<usize, CodecError> {
+        Err(CodecError::DecodeFailed(
+            "codec2-sys not yet linked".to_string(),
+        ))
+    }
+
+    fn decode_lost(&mut self, pcm: &mut [i16]) -> Result<usize, CodecError> {
+        let samples = self.frame_samples();
+        let n = samples.min(pcm.len());
+        // Fill with silence as basic PLC
+        pcm[..n].fill(0);
+        Ok(n)
+    }
+
+    fn codec_id(&self) -> CodecId {
+        self.codec_id
+    }
+
+    fn set_profile(&mut self, profile: QualityProfile) -> Result<(), CodecError> {
+        match profile.codec {
+            CodecId::Codec2_3200 | CodecId::Codec2_1200 => {
+                self.codec_id = profile.codec;
+                self.frame_duration_ms = profile.frame_duration_ms;
+                Ok(())
+            }
+            other => Err(CodecError::UnsupportedTransition {
+                from: self.codec_id,
+                to: other,
+            }),
+        }
+    }
+}