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/Cargo.toml

@@ -10,8 +10,17 @@ description = "WarzonePhone audio codec layer — Opus + Codec2 encoding/decodin
 wzp-proto = { workspace = true }
 tracing = { workspace = true }
 
-# Opus bindings
-audiopus = { workspace = true }
+# Opus bindings — libopus 1.5.2.
+# opusic-c for the encoder (set_dred_duration lives here in Phase 1).
+# opusic-sys for the decoder — we wrap the raw *mut OpusDecoder ourselves
+# because opusic-c::Decoder.inner is pub(crate), blocking the unified
+# decoder + DRED path we need in Phase 3.
+opusic-c = { workspace = true }
+opusic-sys = { workspace = true }
+
+# Zero-cost slice reinterpretation for the i16 ↔ u16 boundary between
+# our PCM buffers and opusic-c's encode API.
+bytemuck = { workspace = true }
 
 # Pure-Rust Codec2 implementation
 codec2 = { workspace = true }