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 84.13043% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (10397d9) to head (469fda8).

Files with missing lines	Patch %	Lines
decoder.go	82.73%	15 Missing and 9 partials ⚠️
internal/resample/silk/down_fir.go	75.26%	23 Missing ⚠️
internal/resample/silk/resampler.go	83.19%	17 Missing and 3 partials ⚠️
internal/resample/silk/fixed.go	71.42%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   76.32%   78.98%   +2.66%     
==========================================
  Files           8       13       +5     
  Lines        1246     1680     +434     
==========================================
+ Hits          951     1327     +376     
- Misses        264      310      +46     
- Partials       31       43      +12     

Flag	Coverage Δ
go	78.98% <84.13%> (+2.66%)	⬆️

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