Only embed watermark chips in STFT bins where the audio signal provides
sufficient masking. Bins in spectral valleys (>25 dB below local peak
within ±4 bins) are skipped — the watermark would be audible there and
they contribute more carrier noise than signal to the correlation.

PAFM is applied in the encoder only. The decoder correlates all bins
unconditionally, because the FM channel alters the spectral shape —
masking decisions made at the encoder do not match the receiver's
spectrum. Skipped bins contribute zero watermark energy (the encoder
didn't modify them) and only carrier noise, which the cepstrum filter
already suppresses by ~6 dB.

On average ~60-70% of bins carry watermark energy per frame, matching
Kirovski's observation. The remaining bins are silent (multiplicative
embedding: magnitude × 1.0 = unchanged).

Over-the-air result (62-minute recording):
 avg|c| = 6286 (27 WM cycles averaged)
 BER = 0/128
 Erasures = 0