ReactWindowDataset: prominent contact-rich window count + stats attribute (ds.stats) + per-filter rejection breakdown in the summary print. Demo: write H.264 MP4 per picked window (was just static PNG); --no_mp4 to skip; --mp4_fps to tune.

ReactWindowDataset: default min_motion_fraction 0.5 -> 0.25 (a window passes if >=25 % of frames per active sensor are above the speed threshold).

ReactWindowDataset: default min_motion_mps 0.03 -> 0.01 (30 mm/s -> 10 mm/s). Recipes + README + demo + CLI updated.

examples/: upgraded ReactWindowDataset (adds motion filter to catch operator-paused windows the bad_frames check cant); canonical self-contained demo_react_window.py (no twm dependency); new examples/README.md with four-filter explanation and recipes per use case

Add example dataloader: ReactWindowDataset for short contact-rich window sampling + demo (static grid + GIF) with sensor-frame overlay