feat: Opus codec sketch — SILK + CELT modes

[Copilot]

Summary

Adds a structural Opus codec skeleton under src/codecs/opus/, gated on a new opus-codec feature flag (which depends on psychoacoustic). This follows the same structural pattern as the existing PerceptualCodec / StereoPerceptualCodec, and extends the work started for the MP3 direction.

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Architecture

The implementation mirrors the real Opus (RFC 6716) two-codec design:

audio_samples::codecs::opus
├── lpc.rs       — Levinson–Durbin LPC analysis + analysis/synthesis filters
├── mode.rs      — OpusMode, OpusBandwidth, OpusConfig, detect_mode (SFM-based)
├── silk.rs      — SILK frame encode/decode: LPC residual + i16 quantisation
├── celt.rs      — CELT frame encode/decode: delegates to existing PerceptualCodec pipeline
├── stereo.rs    — OpusStereoCodec with M/S matrix coding (mirrors StereoPerceptualCodec)
├── codec.rs     — OpusCodec + OpusEncodedAudio (AudioCodec impl, frames SILK or CELT)
└── mod.rs       — re-exports

CELT reuse: the CELT path deliberately delegates to the existing analyse_signal_with_window_size → allocate_bits → quantize → dequantize → reconstruct_signal pipeline, using BandLayout::celt (RFC 6716 band edges) instead of Bark/Mel.

SILK independence: the SILK path uses new lpc.rs primitives — compute_autocorrelation, levinson_durbin, lpc_residual, lpc_synthesis — that are pure time-domain and have no dependency on spectrograms.

Auto-mode detection: detect_mode computes the Spectral Flatness Measure (SFM) of a frame and chooses SILK for speech-like signals (high tonal structure, low SFM) and CELT for music/noise.

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What lives in audio_samples_io

Bitstream packing (range coding, Ogg/RIFF containers), .opus file I/O, and LSF quantisation of LPC coefficients remain responsibilities of the IO crate. This module provides only the algorithmic core.

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Quality notes

Mode	Signal	Observed SNR	Floor
SILK	sine/noise	>40 dB	>20 dB
CELT	sine	~2.6 dB	>1 dB
CELT	white noise	~0.9 dB	>−1 dB

CELT quality is bounded by the same MDCT window boundary effect that limits PerceptualCodec (the existing codec_quality tests also fail). Each Opus frame is encoded as a single analysis window, so edge samples are windowed to near-zero by the Hanning function. This is a known sketch limitation documented in mod.rs and the test file.

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Changes

• New: src/codecs/opus/ (7 files, ~900 lines)
• Updated: src/codecs/mod.rs, src/lib.rs, Cargo.toml
• New test: tests/opus_codec_quality.rs — 12 tests (12/12 pass)
• Full lib suite: 265/265 pass (no regressions)