feat(codec): Phase 3b — CallDecoder DRED reconstruction on packet loss

Phase 3b of the DRED integration — wires the Phase 3a FFI primitives
into the desktop receive path. When the jitter buffer reports a missing
Opus frame, CallDecoder now attempts to reconstruct the audio from the
most recently parsed DRED side-channel state before falling through to
classical PLC.

Architectural refinement vs the PRD's literal wording: the PRD said
"jitter buffer takes a Box<dyn DredReconstructor>". After checking deps,
wzp-transport depends only on wzp-proto (not wzp-codec). Putting DRED
state in the jitter buffer would require a new cross-crate dep and
couple the codec-agnostic buffer to libopus. Instead, this commit keeps
the DRED state ring and reconstruction dispatch inside CallDecoder (one
layer up from the jitter buffer), intercepting the existing
PlayoutResult::Missing signal. Same lookahead/backfill semantics,
cleaner layering, zero change to wzp-transport.

Changes:

  CallDecoder field type: Box<dyn AudioDecoder> → AdaptiveDecoder.
  Required because Phase 3b calls the inherent reconstruct_from_dred
  method, which cannot live on the AudioDecoder trait without dragging
  libopus DredState through wzp-proto. In practice AdaptiveDecoder was
  the only AudioDecoder implementor anyway — the trait abstraction was
  buying nothing. Method call sites unchanged because AdaptiveDecoder
  also implements AudioDecoder.

  New CallDecoder fields:
    - dred_decoder: DredDecoderHandle
    - dred_parse_scratch: DredState  (scratch for parse_into)
    - last_good_dred: DredState      (cached most-recent valid state)
    - last_good_dred_seq: Option<u16>
    - dred_reconstructions: u64      (Phase 4 telemetry)
    - classical_plc_invocations: u64 (Phase 4 telemetry)

  CallDecoder::ingest — on Opus non-repair packets, parse DRED into the
  scratch state. On success (samples_available > 0), std::mem::swap the
  scratch into last_good_dred and record the seq. This is O(1) per
  packet, zero allocation after construction (the two DredState buffers
  are allocated once in new() and reused forever).

  CallDecoder::decode_next — on PlayoutResult::Missing(seq) for Opus
  profiles: if last_good_dred_seq > seq and the seq delta × frame_samples
  fits within samples_available, call audio_dec.reconstruct_from_dred
  and bump dred_reconstructions. Otherwise fall through to classical
  PLC and bump classical_plc_invocations. The Codec2 path always falls
  through to classical PLC since DRED is libopus-only and
  AdaptiveDecoder::reconstruct_from_dred rejects Codec2 tiers
  explicitly.

  OpusDecoder and AdaptiveDecoder: new inherent reconstruct_from_dred
  method that delegates to the underlying DecoderHandle. Needed to
  bridge CallDecoder's wzp-client code to the Phase 3a FFI wrappers
  without touching the AudioDecoder trait.

CRITICAL FINDING — raised DRED loss floor from 5% to 15%:

Phase 3b testing discovered that libopus 1.5's DRED emission window
scales aggressively with OPUS_SET_PACKET_LOSS_PERC. Empirical data
(see probe_dred_samples_available_by_loss_floor, an #[ignore]'d
diagnostic test in call.rs):

  loss_pct   samples_available   effective_ms
    5%        720                  15 ms  (useless!)
   10%        2640                 55 ms
   15%        4560                 95 ms
   20%        6480                135 ms
   25%+       8400 (capped)       175 ms  (~87% of 200 ms configured)

The Phase 1 default of 5% produced only a 15 ms reconstruction window
— too small to even cover a single 20 ms Opus frame. DRED was
effectively disabled even though it was emitting bytes. Raised the
floor to 15% (95 ms window) as the minimum that actually provides
single-frame loss recovery. This updates Phase 1's DRED_LOSS_FLOOR_PCT
constant in opus_enc.rs and the accompanying module docstring.

Trade-off: 15% assumed loss slightly increases encoder bitrate overhead
on clean networks. Measured via the existing phase1 bitrate probe:

  Before (5% floor):  3649 bytes/sec at Opus 24k + 300 Hz sine
  After  (15% floor): 3568 bytes/sec at Opus 24k + 300 Hz sine

The delta is within noise — 15% isn't meaningfully more expensive than
5% on this signal, which suggests the DRED emission size is signal-
dependent rather than loss-dependent for small values. Net result: we
get a 6x larger reconstruction window for essentially free.

Tests (+3 DRED recovery, +1 #[ignore]'d probe):
- opus_single_packet_loss_is_recovered_via_dred — full encode → ingest
  → decode_next loop with one packet dropped mid-stream. Asserts
  dred_reconstructions ≥ 1 and observes the exact counter deltas.
- opus_lossless_ingest_never_triggers_dred_or_plc — baseline behavior,
  lossless stream never takes the Missing branch.
- codec2_loss_falls_through_to_classical_plc — Codec2 never
  reconstructs via DRED even if state were populated (which it won't
  be — Codec2 packets don't carry DRED bytes).
- probe_dred_samples_available_by_loss_floor — #[ignore]'d diagnostic
  that sweeps loss_pct values and prints the resulting DRED window
  sizes. Kept for future tuning work.

New CallDecoder introspection accessors (public but undocumented in
the PRD): last_good_dred_seq() and last_good_dred_samples_available()
for test diagnostics and future telemetry surfaces in Phase 4.

Verification:
- cargo check --workspace: zero errors
- cargo test -p wzp-codec --lib: 68 passing (Phase 3a baseline held)
- cargo test -p wzp-client --lib: 35 passing (+3 Phase 3b tests,
  +1 ignored diagnostic, no regressions)

Next up: Phase 3c mirrors this on the Android engine.rs receive path.

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

crates/wzp-codec/src/opus_dec.rs

@@ -6,7 +6,7 @@
 //! `opus_decoder_dred_decode`. See `dred_ffi.rs` for the rationale and
 //! `docs/PRD-dred-integration.md` for the full plan.
 
-use crate::dred_ffi::DecoderHandle;
+use crate::dred_ffi::{DecoderHandle, DredState};
 use wzp_proto::{AudioDecoder, CodecError, CodecId, QualityProfile};
 
 /// Opus decoder implementing [`AudioDecoder`].
@@ -36,6 +36,24 @@ impl OpusDecoder {
     pub fn frame_samples(&self) -> usize {
         (48_000 * self.frame_duration_ms as usize) / 1000
     }
+
+    /// Reconstruct a lost frame from a previously parsed `DredState`.
+    ///
+    /// Phase 3b entry point: callers (CallDecoder / engine.rs) use this to
+    /// synthesize audio for gaps detected by the jitter buffer when DRED
+    /// side-channel state from a later-arriving packet covers the gap's
+    /// sample offset. `offset_samples` is measured backward from the anchor
+    /// packet that produced `state`. See `DecoderHandle::reconstruct_from_dred`
+    /// for the full semantics.
+    pub fn reconstruct_from_dred(
+        &mut self,
+        state: &DredState,
+        offset_samples: i32,
+        output: &mut [i16],
+    ) -> Result<usize, CodecError> {
+        self.inner
+            .reconstruct_from_dred(state, offset_samples, output)
+    }
 }
 
 impl AudioDecoder for OpusDecoder {