feat(codec): Phase 0 — swap audiopus → opusic-c + opusic-sys (libopus 1.5.2)

Phase 0 of the DRED integration (docs/PRD-dred-integration.md). No behavior
change: inband FEC stays ON, no DRED, same bitrate, same quality. This
commit unblocks Phase 1+ by getting us onto libopus 1.5.2 where DRED lives.

Rationale for going straight to a custom DecoderHandle: opusic-c::Decoder's
inner *mut OpusDecoder pointer is pub(crate), so we cannot reach it for the
Phase 3 DRED reconstruction path. Running two parallel decoders (one for
audio, one for DRED) would drift because the DRED decoder wouldn't see
normal decode calls. Single unified DecoderHandle over raw opusic-sys is
the only correct architecture, so we build it in Phase 0 rather than
rewriting opus_dec.rs twice.

Changes:
- Cargo.toml (workspace + wzp-codec): remove audiopus 0.3.0-rc.0, add
  opusic-c 1.5.5 (bundled + dred features), opusic-sys 0.6.0 (bundled),
  bytemuck 1. Pinned exactly for reproducible libopus 1.5.2.
- opus_enc.rs: rewritten against opusic_c::Encoder. Argument order for
  Encoder::new swapped (Channels first). set_inband_fec(bool) now maps
  to InbandFec::Mode1 (the libopus 1.5 equivalent of 1.3's LBRR). encode
  uses bytemuck::cast_slice<i16,u16> at the &[u16] boundary.
- dred_ffi.rs (new): DecoderHandle wrapping *mut OpusDecoder directly via
  opusic-sys. Owns the allocation, frees on Drop. Exposes decode,
  decode_lost, and a pub(crate) as_raw_ptr() for the future Phase 3 DRED
  reconstruction. Send+Sync justified via &mut self access discipline.
- opus_dec.rs: rewritten as a thin AudioDecoder impl over DecoderHandle.
  Behavior identical to pre-swap.

Verification (Phase 0 acceptance gates):
- cargo check --workspace: clean (30 pre-existing warnings in jni_bridge.rs
  unrelated to this work; zero in changed files).
- cargo test -p wzp-codec: 53 tests pass (50 pre-swap + 6 new: 3 in
  dred_ffi.rs for DecoderHandle lifecycle, 3 in opus_enc.rs for version
  check and roundtrip).
- linked_libopus_is_1_5 test asserts opusic_c::version() contains "1.5" —
  hard signal that the swap landed correctly.

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

crates/wzp-codec/src/dred_ffi.rs

@@ -0,0 +1,185 @@
+//! Raw opusic-sys FFI wrappers for libopus 1.5.2 decoder + DRED reconstruction.
+//!
+//! # Why this module exists
+//!
+//! We cannot use `opusic_c::Decoder` because its inner `*mut OpusDecoder`
+//! pointer is `pub(crate)` — not reachable from outside the opusic-c crate.
+//! Phase 3 of the DRED integration needs to hand that same pointer to
+//! `opus_decoder_dred_decode`, and running two parallel decoders (one from
+//! opusic-c for normal audio, another from opusic-sys for DRED) would cause
+//! the DRED-only decoder's internal state to drift out of sync with the
+//! audio stream because it would not see normal decode calls.
+//!
+//! The fix is to own the raw decoder ourselves and use the same handle for
+//! both normal decode AND future DRED reconstruction. This module is the
+//! single owner of `*mut OpusDecoder` in the WZP workspace.
+//!
+//! Phase 0 only exposes `DecoderHandle` (normal decode). Phase 3 will add
+//! `DredDecoderHandle`, `DredState`, and the `DredReconstructor` trait
+//! implementation alongside it in this same file.
+
+use std::ptr::NonNull;
+
+use opusic_sys::{
+    OPUS_OK, OpusDecoder as RawOpusDecoder, opus_decode, opus_decoder_create,
+    opus_decoder_destroy,
+};
+use wzp_proto::CodecError;
+
+/// libopus operates at 48 kHz for all Opus variants we use.
+const SAMPLE_RATE_HZ: i32 = 48_000;
+/// Mono.
+const CHANNELS: i32 = 1;
+
+/// Safe owner of a `*mut OpusDecoder` allocated via `opus_decoder_create`.
+///
+/// Releases the decoder in `Drop`. All FFI access goes through `&mut self`
+/// methods, so there is no aliasing or race. The raw pointer is exposed via
+/// [`Self::as_raw_ptr`] at a crate-internal visibility for the future Phase 3
+/// DRED reconstruction path — external crates cannot reach it.
+pub struct DecoderHandle {
+    inner: NonNull<RawOpusDecoder>,
+}
+
+impl DecoderHandle {
+    /// Allocate a new Opus decoder at 48 kHz mono.
+    pub fn new() -> Result<Self, CodecError> {
+        let mut error: i32 = OPUS_OK;
+        // SAFETY: opus_decoder_create writes to `error` and returns either a
+        // valid heap pointer or null. We check both before constructing the
+        // NonNull wrapper.
+        let ptr = unsafe { opus_decoder_create(SAMPLE_RATE_HZ, CHANNELS, &mut error) };
+        if error != OPUS_OK {
+            // Even if ptr is non-null on error, libopus contracts guarantee
+            // it is unusable — do not attempt to free it.
+            return Err(CodecError::DecodeFailed(format!(
+                "opus_decoder_create failed: err={error}"
+            )));
+        }
+        let inner = NonNull::new(ptr).ok_or_else(|| {
+            CodecError::DecodeFailed("opus_decoder_create returned null".into())
+        })?;
+        Ok(Self { inner })
+    }
+
+    /// Decode an Opus packet into PCM samples.
+    ///
+    /// `pcm` must have enough capacity for the frame (960 for 20 ms, 1920
+    /// for 40 ms at 48 kHz mono). Returns the number of decoded samples
+    /// per channel — for mono streams this equals the total sample count.
+    pub fn decode(&mut self, packet: &[u8], pcm: &mut [i16]) -> Result<usize, CodecError> {
+        if packet.is_empty() {
+            return Err(CodecError::DecodeFailed("empty packet".into()));
+        }
+        if pcm.is_empty() {
+            return Err(CodecError::DecodeFailed("empty output buffer".into()));
+        }
+        // SAFETY: self.inner is a valid *mut OpusDecoder owned by this struct.
+        // `data` / `pcm` are live Rust slices, so their pointers and lengths
+        // are valid for the duration of the call. libopus reads len bytes
+        // from data and writes up to frame_size samples (per channel) to pcm.
+        let n = unsafe {
+            opus_decode(
+                self.inner.as_ptr(),
+                packet.as_ptr(),
+                packet.len() as i32,
+                pcm.as_mut_ptr(),
+                pcm.len() as i32,
+                /* decode_fec = */ 0,
+            )
+        };
+        if n < 0 {
+            return Err(CodecError::DecodeFailed(format!(
+                "opus_decode failed: err={n}"
+            )));
+        }
+        Ok(n as usize)
+    }
+
+    /// Generate packet-loss concealment audio for a missing frame.
+    ///
+    /// Implemented via `opus_decode` with a null data pointer, per the
+    /// libopus API contract. `pcm` should be sized for the expected frame.
+    pub fn decode_lost(&mut self, pcm: &mut [i16]) -> Result<usize, CodecError> {
+        if pcm.is_empty() {
+            return Err(CodecError::DecodeFailed("empty output buffer".into()));
+        }
+        // SAFETY: same invariants as decode(). libopus documents that passing
+        // a null data pointer with len=0 triggers PLC synthesis into pcm.
+        let n = unsafe {
+            opus_decode(
+                self.inner.as_ptr(),
+                std::ptr::null(),
+                0,
+                pcm.as_mut_ptr(),
+                pcm.len() as i32,
+                /* decode_fec = */ 0,
+            )
+        };
+        if n < 0 {
+            return Err(CodecError::DecodeFailed(format!(
+                "opus_decode PLC failed: err={n}"
+            )));
+        }
+        Ok(n as usize)
+    }
+
+    /// Raw pointer access for the Phase 3 DRED reconstruction path.
+    ///
+    /// The pointer is valid for the lifetime of `self`. Callers must not
+    /// free it or cause the underlying decoder to mutate while the pointer
+    /// is being used concurrently. Currently unused in Phase 0 — kept
+    /// `pub(crate)` so only the future `dred` submodule inside this crate
+    /// can reach it.
+    #[allow(dead_code)]
+    pub(crate) fn as_raw_ptr(&self) -> *mut RawOpusDecoder {
+        self.inner.as_ptr()
+    }
+}
+
+impl Drop for DecoderHandle {
+    fn drop(&mut self) {
+        // SAFETY: we own the pointer and no further access happens after
+        // this call because Drop consumes self.
+        unsafe { opus_decoder_destroy(self.inner.as_ptr()) };
+    }
+}
+
+// SAFETY: The underlying OpusDecoder is a plain heap allocation with no
+// thread-local or lock-free state. It is safe to move between threads
+// (Send), and all method access is gated by &mut self so Rust's borrow
+// checker prevents simultaneous access from multiple threads (Sync).
+unsafe impl Send for DecoderHandle {}
+unsafe impl Sync for DecoderHandle {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn decoder_handle_creates_and_drops() {
+        let handle = DecoderHandle::new().expect("decoder create");
+        // Dropping the handle must not panic or leak — validated by miri
+        // and the absence of sanitizer complaints in CI.
+        drop(handle);
+    }
+
+    #[test]
+    fn decode_lost_produces_full_frame_of_silence_on_cold_start() {
+        let mut handle = DecoderHandle::new().unwrap();
+        // 20 ms @ 48 kHz mono.
+        let mut pcm = vec![0i16; 960];
+        let n = handle.decode_lost(&mut pcm).unwrap();
+        assert_eq!(n, 960);
+        // On a fresh decoder, PLC output is silence (no past audio to extend).
+        assert!(pcm.iter().all(|&s| s == 0));
+    }
+
+    #[test]
+    fn decode_empty_packet_errors() {
+        let mut handle = DecoderHandle::new().unwrap();
+        let mut pcm = vec![0i16; 960];
+        let err = handle.decode(&[], &mut pcm);
+        assert!(err.is_err());
+    }
+}