Add SILK VAD (voice activity detection)

[thomas-vilte]

Description

ports the SILK voice activity detector: speech activity estimation (VAD.c's silk_VAD_GetSA_Q8), the per-band noise-level tracker (silk_VAD_GetNoiseLevels), the 2-band analysis filter bank (ana_filt_bank_1.c), and the pitch-adaptive high-pass cutoff (HP_variable_cutoff.c). Only depends on the math primitives from #153, nothing else

went through it function by function against the real C source and it lines up exactly — every constant, shift and branch. the first test pass left a few branches uncovered though: the high-energy division path in the SNR ratio calc, the mid-power-level scaling branch, and the decreasing-pitch case in the cutoff adaptation. one of those (the mid-power branch) needed a specific amplitude to land in — ended up temporarily instrumenting the function with a print statement to see the actual internal value across a few amplitudes, found one that lands there reliably, then pulled the print back out. all five functions are at 100% now

Reference issue

part of the SILK encoder series (#147)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.40%. Comparing base (4bed3b6) to head (fcc434b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
+ Coverage   88.19%   88.40%   +0.20%     
==========================================
  Files          34       35       +1     
  Lines        7868     8010     +142     
==========================================
+ Hits         6939     7081     +142     
  Misses        703      703              
  Partials      226      226              

Flag	Coverage Δ
go	88.40% <100.00%> (+0.20%)	⬆️

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