corpus_structural.py drives render48k (48k/4096 structural chain) across the
full corpus with bridge-compare mode. Results: comb 6.12 vs bridge 10.15
(structural wins multiband already), res better, single-band worse; dual@500
constant -6.7dB q-independent over-cut exposes missing reduction saturation.
Param sweep (gamma x MULT, verified builds) confirms current optimum; curve
SHAPE is the gap -> Step 7 capture or bigkernel input re-examination.
