Add SILK noise shape primitives

[thomas-vilte]

Description

adds two small standalone pieces:

• noise_shape.go: sigmoid and warpedAutocorrelationFLP — autocorrelation through a chain of first-order all-pass warping sections, used by the noise-shaping analysis stage.
• control_snr.go: controlSNR — target residual-quantizer SNR as a function of bitrate, plus the NB/MB/WB bitrate→SNR lookup tables

ported from SigProc_FLP.h, warped_autocorrelation_FLP.c, and control_SNR.c. control_snr.go didn't have a test upstream, so I wrote one from scratch (low-rate floor, first reachable table bucket, high-rate clamp, and the 10ms subframe-count rate offset)

this is a smaller slice of noise shaping than originally planned — noise_shape_analysis.go (the actual silk_noise_shape_analysis_FLP/silk_process_gains_FLP orchestration) turned out to be entirely *Encoder methods with no separable pure logic underneath, so the whole file — plus a couple of small helpers only those two functions use — moves to the piece that introduces the encoder struct instead

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (12f8137) to head (1100487).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   89.27%   89.31%   +0.04%     
==========================================
  Files          44       46       +2     
  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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