Add SILK NLSF quantization

[thomas-vilte]

Description

adds the two-stage NLSF quantizer (silk_NLSF_encode): quantizeNLSF searches every stage-1 codebook vector, greedily quantizes the stage-2 residual backwards for each, and keeps whichever candidate gives the lowest weighted distortion after reconstructing and stabilizing it the way the decoder will. stabilizeNLSF is the encoder-side port of the same minimum-spacing procedure the decoder already had — I refactored decoder.go's normalizeLSFStabilization to call it directly instead of carrying its own duplicate ~140-line copy

ported from NLSF_encode.c and NLSF_stabilize.c

left encodeNLSF/emitNLSFIndices out of this one, same reasoning as find_pitch_lags.go in #158 — both are methods on *Encoder (need rangeEncoder), and that type doesn't exist yet. What's here is the quantization math itself; the range-coding wiring lands with the rest of the encoder plumbing. Test-wise this meant swapping the original's full Encoder→bitstream→Decoder round trip for a direct one — replaying the decoder's stage-2 reconstruction formula against the quantizer's own output, no bitstream involved. The regression test for the stage-2 clamp bug (#147) is unchanged in what it actually checks

known simplification, not introduced here: this quantizer brute-forces every stage-1 codebook entry with self-derived weights, instead of libopus's trellis/delayed-decision search (NLSF_del_dec_quant) with real per-frame perceptual weights and a rate term. Still produces valid, decodable output — just not necessarily the same codebook index libopus would pick

Reference issue

part of the SILK encoder series (#159)

[codecov]

Codecov Report

❌ Patch coverage is 97.56098% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.87%. Comparing base (ad89c91) to head (e3099f7).

Files with missing lines	Patch %	Lines
internal/silk/nlsf_encode.go	97.54%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
+ Coverage   88.57%   88.87%   +0.30%     
==========================================
  Files          38       39       +1     
  Lines        8252     8325      +73     
==========================================
+ Hits         7309     7399      +90     
+ Misses        712      694      -18     
- Partials      231      232       +1     

Flag	Coverage Δ
go	88.87% <97.56%> (+0.30%)	⬆️

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