Claude: refineMotionVectors() — improved debug images

- Masked frames: replace single-frame display with full fmin..fmax stack
  (scene_titles used as slice labels; scroll to watch target stay in place)
- Source frame display removed (not needed alongside masked stack)
- Single-tile corr2d block removed; replaced by full CORR2D multi-slice image
  after the nseq loop — one slice per keyframe, same tiled layout as *CORR2D.tiff,
  slice-labeled from scene_titles, uses corr_border_contrast from CLTParameters
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>