Add SILK LTP analysis and quantization

[thomas-vilte]

Description

adds the long-term-predictor (LTP) pipeline pieces that don't need the encoder struct yet:

• ltp_analysis.go: builds the normalized correlation matrices/vectors per subframe (findLTPFLP, silk_find_LTP_FLP)
• ltp_quant.go: the fixed-point codebook search (vqWMatEC, silk_VQ_WMat_EC) that scores each of the 3 LTP codebooks against a correlation matrix, plus log2lin
• ltp_scale.go: picks the LTP state-scaling index for independently-coded voiced frames based on prediction gain, target SNR, and expected packet loss (silk_LTP_scale_ctrl_FLP)

ported from find_LTP_FLP.c, VQ_WMat_EC.c, and LTP_scale_ctrl_FLP.c

left quantLTPGains out — same reasoning as find_pitch_lags.go (#158) and encodeNLSF (#160): it's a method on *Encoder (threads a gain budget across frames via sumLogGainQ7), and that type doesn't exist yet. What's here is everything it calls (vqWMatEC and the codebook tables), with a direct test built from a real correlation matrix instead of going through the method

Reference issue

part of the SILK encoder series (#160)