Hotfix: fix dataset asset download layout

[BotRunner64]

Summary

• Download dataset assets from data/datasets//.h5 into data/datasets/ instead of the old seed-only layout.
• Clear stale entry cache before placing downloaded assets so old NPZ files are not copied from cache.
• Update training defaults and docs to precompute and train on all downloaded datasets via data/datasets_precomputed.

Tests

• pytest tests/test_download_assets.py tests/test_train_script.py -q
• python -m py_compile scripts/setup/download_assets.py teleopit/runtime/external_assets.py train_mimic/scripts/train.py train_mimic/scripts/play.py train_mimic/scripts/benchmark.py train_mimic/tasks/tracking/config/constants.py