Update README for flattened single-split structure with embedded videos and inline boxes

Remove obsolete loose videos, annotations/clinical_info parquets, and splits (flattened into data/train parquet)

Flatten B: single train split, embedded videos + inline per-box annotations + clinical info, remove splits and frames

Add 173 raw cystoscopy videos

Add 173 raw cystoscopy videos

Add parquet tables, splits, and dataset card

initial commit