Add sub_handcrafted eval set (Quirke handcrafted)

Add add_handcrafted eval set (Quirke handcrafted)

Add dataset configs for HF viewer

Update dataset card with Quirke complexity labels

Remove add_sub_4digit

Remove add_4digit

Update add_sub_6digit: add complexity labels + stratified eval

Update add_6digit: add complexity labels + stratified eval

Add dataset card

Upload add_sub_4digit: add_sub 4-digit, 500000 train / 10000 val

Upload add_4digit: add 4-digit, 500000 train / 10000 val

Upload add_sub_6digit: add_sub 6-digit, 500000 train / 10000 val

Upload add_6digit: add 6-digit, 500000 train / 10000 val

initial commit