Remove orphan test/ folder (test split now lives under 1BT config)

Fix YAML: 1BT config now has train + test splits (exhaustive 4-digit)

Add 1BT: k=4 addition (45,004,481 train + 5,000,519 test examples)

Clear old 1BT before re-upload

Add README

Initial 4-digit stratified dataset (copy of addition_dataset/4digit)

initial commit