Implement SILK stereo decoding

[zshang-oai]

What changed

• Implemented SILK mid-side stereo decoding and stereo unmixing.
• Added channel-aware sample repetition for float32 and s16 output conversion.
• Added stereo iCDF tables and test coverage for stereo weights, mono delay, and interleaved resampling.

[codecov]

Codecov Report

❌ Patch coverage is 84.33180% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.02%. Comparing base (87a3291) to head (781a283).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/silk/decoder.go	83.92%	16 Missing and 11 partials ⚠️
decoder.go	66.66%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   77.51%   79.02%   +1.50%     
==========================================
  Files           8        8              
  Lines        1352     1535     +183     
==========================================
+ Hits         1048     1213     +165     
- Misses        270      276       +6     
- Partials       34       46      +12     

Flag	Coverage Δ
go	79.02% <84.33%> (+1.50%)	⬆️

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[Copilot]

Pull request overview

Adds SILK stereo support to the Opus decoder path, enabling mid/side stereo decoding and ensuring resampling/bitdepth conversion preserves stereo interleaving.

Changes:

• Implement SILK mid/side stereo decoding, mid-only side skipping, and stereo unmixing per RFC 6716.
• Make resampling and float32→s16 conversion channel-aware to preserve interleaved stereo during upsampling.
• Extend/update tests for stereo decoding behavior, stereo iCDF tables, mono delay, and interleaved upsampling.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
decoder.go	Tracks decoded sample count; sizes SILK buffer for stereo; uses channel-aware upsample/bitdepth conversion.
decoder_test.go	Updates internal decode call sites for new return signature.
packet_test.go	Adjusts tests for new decode signature; validates stereo buffer sizing behavior.
internal/silk/decoder.go	Implements stereo weight decode, mid-only flag, mono delay, side decoder state handling, and stereo unmixing.
internal/silk/decoder_test.go	Replaces stereo “TODO” with functional tests; adds targeted coverage for new stereo/mono-delay logic.
internal/silk/icdf.go	Adds stereo iCDF tables (weights + mid-only).
internal/resample/resample.go	Updates Up to be channel-aware and return validation errors.
internal/resample/resample_test.go	Adds tests for mono/stereo interleaved upsampling and validation errors.
internal/bitdepth/bitdepth.go	Makes float32→s16 conversion channel-aware with resample repetition and input validation.
internal/bitdepth/bitdepth_test.go	Adds stereo + resample coverage and validation/error tests.
internal/rangecoding/decoder.go	Comment formatting/alignment changes only.

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[Copilot]

decodeStereo allocates fresh mid and side buffers on every SILK frame (make([]float32, frameSampleCount)), which will add GC pressure in steady-state decoding (packets at 50/100 fps). Consider reusing per-decoder scratch buffers (e.g., fields on Decoder sized to maxSilkFrameSampleCount) or decoding into preallocated slices reused across loop iterations, and only allocating when the required size grows.