Materialize dense/sparse/plain subtask_* columns from lerobot_annotations.json

Fix: remap episode_index to dense 0..N-1, recompute index/dataset_from/to_index

Fix: recompute index/dataset_from_index/dataset_to_index after episode exclusion

Recompute stats.json: 273 eps / 585,425 frames; add subtask_index & task_index_high_level

Re-export: fix subtask_index ordering, exclude [6, 8, 22, 40, 55, 79, 111, 113, 134, 247, 266]