Publish motherboard video release (MP4+parquet, 640x480) + curation + previews

Add per-task camera extrinsics: motherboard=May-12 (RMSE ~5mm), pushT=June-26 (RMSE ~0.6px). Each data/<task>/calibration/ has T_mocap_to_cam_{left,middle,right}.json + T_gel_to_rigid_{left,right}.json + calibration.json manifest (epoch, applies-to dates, RMSE, projection chain).