Port the RFC SILK output resampler

[zshang-oai]

Summary

This ports the decoder-side SILK output resampler from the RFC reference implementation into a dedicated Go subpackage and wires the Opus decoder to resample SILK output directly to the caller's requested output rate.

The branch removes the old naive sample-repetition path and replaces it with the RFC-style decoder resampler state, delay handling, and per-ratio FIR/IIR paths.

Public API

This PR updates the public decoder API so callers can request a specific output sample rate and channel count instead of relying on the old fixed behavior:

• add NewDecoderWithOutput(sampleRate, channels)
• change (*Decoder).Init(sampleRate, channels) to validate and store the requested output format
• add DecodeToInt16 and DecodeToFloat32 helpers that return samples-per-channel
• make Decode/DecodeFloat32 use the configured output format before converting to the public PCM buffers

That aligns the exposed API with the output rates exercised by the RFC conformance flow: 8, 12, 16, 24, or 48 kHz, mono or stereo.

RFC Alignment

Where possible, this follows the RFC/reference implementation directly:

• RFC 6716 Section 4.2.9 defines decoder-side SILK resampling and makes the resampler delay normative while allowing implementation freedom in the filter itself.
• RFC 6716 Section 6.1 defines conformance against the Appendix A reference decoder and explicitly evaluates supported output sampling rates and channel counts.
• RFC 6716 says the Appendix A reference source takes precedence if prose and source diverge, so this branch stays structurally close to the reference resampler and decoder-side setup.

Relevant references:

• RFC 6716 Section 4.2.9
• RFC 6716 Section 6.1
• RFC 6716 Appendix A

Implementation Notes

• add internal/resample/silk as a Go port of the RFC SILK decoder resampler
• preserve the reference delay matrix and supported decoder-side rate conversions
• keep per-channel resampler state in Decoder so mono/stereo SILK output can be resampled without reinitializing every packet
• update PCM conversion helpers to operate on configured channel counts
• add impulse fixtures and round-trip checks for the supported decoder-side SILK ratios

Validation

• go test ./...
• go test -race ./...
• attempted golangci-lint run, but the tool currently fails in this environment before analysis with context loading failed: no go files to analyze

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.46%. Comparing base (914b0f4) to head (50484c7).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/resample/silk/resampler.go	79.56%	26 Missing and 2 partials ⚠️
internal/resample/silk/down_fir.go	76.04%	23 Missing ⚠️
decoder.go	84.37%	12 Missing and 8 partials ⚠️
internal/resample/silk/fixed.go	71.42%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
+ Coverage   79.02%   80.46%   +1.44%     
==========================================
  Files           8       13       +5     
  Lines        1535     1966     +431     
==========================================
+ Hits         1213     1582     +369     
- Misses        276      329      +53     
- Partials       46       55       +9     

Flag	Coverage Δ
go	80.46% <83.33%> (+1.44%)	⬆️

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[Copilot]

Pull request overview

Ports the RFC 6716 decoder-side SILK output resampler into Go and updates the Opus decoder to produce PCM at a caller-configured output sample rate and channel count (instead of the previous naive upsample-by-repetition path).

Changes:

• Add internal/resample/silk implementing the SILK decoder resampler (delay handling + per-ratio FIR/IIR paths) with fixture-based tests.
• Update Decoder to accept a configured output format (NewDecoderWithOutput / Init) and resample SILK output accordingly.
• Add DecodeToInt16 / DecodeToFloat32 helpers and adjust PCM conversion rounding/clamping behavior.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
decoder.go	Stores requested output format, resamples SILK output to requested rate/channels, adds decode helpers.
errors.go	Adds new public decoder validation/buffer errors.
decoder_test.go	Adds tests for new constructor and decode helper behavior.
packet_test.go	Updates empty-packet decode test for expanded return values.
internal/resample/silk/resampler.go	Core resampler state, init, delay management, and float/int conversions.
internal/resample/silk/up2_hq.go	2x HQ upsampling path.
internal/resample/silk/iir_fir.go	IIR/FIR upsample/interpolation path.
internal/resample/silk/down_fir.go	Downsample FIR path.
internal/resample/silk/ar2.go	AR2 helper used by downsampling.
internal/resample/silk/fixed.go	Fixed-point math helpers used by resampler.
internal/resample/silk/rom.go	Resampler coefficient ROM tables.
internal/resample/silk/resampler_test.go	Impulse fixtures, chunking equivalence, and round-trip error bounds.
internal/bitdepth/bitdepth.go	Adjusts float32→int16 quantization (round/clamp).
internal/bitdepth/bitdepth_test.go	Updates expected byte output for new quantization.
internal/resample/resample.go	Removes old naive repetition-based resampler implementation.
internal/resample/resample_test.go	Removes tests for the deleted naive resampler.

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