Add SILK gain quantization

[thomas-vilte]

Description

adds quantizeGains — per-subframe log-domain gain quantization with hysteresis (silk_gains_quant): independent 6-bit gain index for the first subframe of a frame (when not delta-coded), delta index otherwise, with a double-step mechanism so large gain jumps can still be reached within the delta range

ported from gain_quant.c

this one needed a small structural change, not just a straight extraction: quantizeGains was a method on *Encoder (kept the running previousLogGain on the struct), but that type doesn't exist in this package yet — same blocker as find_pitch_lags.go/encodeNLSF/quantLTPGains before it. Instead of deferring the whole file (there'd be nothing left for this PR), I turned it into a plain function that takes previousLogGain as an explicit *int32 in/out parameter, mirroring the pattern pitchAnalysisCore already uses for ltpCorr in this same package. encodeSubframeGains/emitGainIndices (the range-coder wiring) are what's still deferred to the orchestration piece

tests replay the decoder's reconstruction math directly against the returned indices instead of a full Encoder→bitstream→Decoder round trip, same approach as the NLSF piece (#160)

Reference issue

part of the SILK encoder series (#160)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (12f8137) to head (e573bce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   89.27%   89.31%   +0.04%     
==========================================
  Files          44       45       +1     
  Lines        8783     8817      +34     
==========================================
+ Hits         7841     7875      +34     
  Misses        707      707              
  Partials      235      235              

Flag	Coverage Δ
go	89.31% <100.00%> (+0.04%)	⬆️

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