Add SILK pitch analysis core

[thomas-vilte]

Description

adds pitchAnalysisCore — the two-stage pitch estimator (silk_pitch_analysis_core_FLP): a coarse search at 4 kHz over the full lag range, followed by a refined search at 8 kHz (and, for 16 kHz input, a stage-3 refinement back in the original signal) using the codebook tables in pitch_est_tables.c. Also includes the two escape paths (low first-stage correlation, no codebook candidate above threshold) and the previous-lag continuity bias that favors sticking with the prior frame's pitch

ported from pitch_analysis_core_FLP.c and pitch_est_tables.c/pitch_est_defines.h

left find_pitch_lags.go out of this one even though the original split plan grouped it here — it's a method on *Encoder (needs previousLag, ltpCorr, etc.), and that type doesn't exist in this package yet. Moving it to land alongside find_pred_coefs.go, once there's an encoder to hang it off of

one scope note carried over from the original code: only 8 and 16 kHz are handled today (12 kHz needs resamplerDown2_3, which isn't ported yet) — this matches the upstream file as-is, not something reduced during the split

Reference issue

part of the SILK encoder series (#157)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.91%. Comparing base (ad89c91) to head (7689d8e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   88.57%   88.91%   +0.34%     
==========================================
  Files          38       39       +1     
  Lines        8252     8506     +254     
==========================================
+ Hits         7309     7563     +254     
  Misses        712      712              
  Partials      231      231              

Flag	Coverage Δ
go	88.91% <100.00%> (+0.34%)	⬆️

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