Add SILK float pitch/resampler primitives

[thomas-vilte]

Description

adds the last few shared floating-point primitives used by the pitch analysis stage: resamplerDown2 (2:1 decimator), float2ShortArray/short2FloatArray, energyFLP, pitchXcorr, and the two insertion sort helpers (insertionSortDecreasingFLP/insertionSortIncreasing). these sit alongside the LPC float primitives from #154 — same "shared DSP building blocks" piece of the split, just the half that didn't make it into that PR

ported from resampler_down2.c (coefficients cross-checked against resampler_rom.h), the float/short conversion inlines in SigProc_FLP.h, energy_FLP.c, and sort_FLP.c/sort.c

one thing worth flagging: pitchXcorr here is a plain loop over innerProductFLP, not libopus's SIMD-optimized celt_pitch_xcorr. Same result, just without the vectorized fast path — fine for a shared primitive, the actual pitch-analysis algorithm that calls it is a separate, later piece of this split

Reference issue

part of the SILK encoder series (#154)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.58%. Comparing base (4221339) to head (c74dad0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   88.49%   88.58%   +0.09%     
==========================================
  Files          36       37       +1     
  Lines        8074     8141      +67     
==========================================
+ Hits         7145     7212      +67     
  Misses        703      703              
  Partials      226      226              

Flag	Coverage Δ
go	88.58% <100.00%> (+0.09%)	⬆️

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