Add SILK LPC-to-NLSF conversion (a2nlsf)

[thomas-vilte]

Description

adds silk_A2NLSF — converts monic LPC/whitening filter coefficients (Q16) to Normalized Line Spectral Frequencies (Q15). This is root-finding: the filter's even/odd polynomials get evaluated on a fixed cosine grid, with binary-division refinement around each sign crossing, alternating between the two polynomials for each successive root. also includes the float wrapper (a2nlsfFLP) and the bwexpander32 bandwidth-expansion helper used when the root search doesn't converge on the first pass

ported from A2NLSF.c, bwexpander_32.c, and the FLP wrapper in wrappers_FLP.c

one thing worth calling out: the "ran out of retries, fall back to an evenly-spaced white spectrum" branch (maxIterA2NLSF exhausted) is real defensive code in libopus, but I couldn't construct an input that reaches it — threw millions of random and clustered/near-duplicate NLSF vectors at it and the worst case I found needed 16 of the 17 allowed retries. left it uncovered rather than force a contrived case; the rest of the file is in the high 90s/100%

Reference issue

part of the SILK encoder series (#156)

[codecov]

Codecov Report

❌ Patch coverage is 87.38739% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.47%. Comparing base (4221339) to head (30eb2f8).

Files with missing lines	Patch %	Lines
internal/silk/a2nlsf.go	87.38%	9 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #157      +/-   ##
==========================================
- Coverage   88.49%   88.47%   -0.02%     
==========================================
  Files          36       37       +1     
  Lines        8074     8185     +111     
==========================================
+ Hits         7145     7242      +97     
- Misses        703      712       +9     
- Partials      226      231       +5     

Flag	Coverage Δ
go	88.47% <87.38%> (-0.02%)	⬇️

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