feat: add Street Fighter, Terraria, and Space Invaders datasets

[tashapais]

Summary

• Adds StreetFighterDataset, TerrariaDataset, and SpaceInvadersDataset subclasses in datasets/datasets.py, following the same pattern as existing dataset classes
• Adds load_street_fighter, load_terraria, and load_space_invaders loader functions in datasets/data_utils.py
• Adds STREET_FIGHTER, TERRARIA, and SPACE_INVADERS dispatch cases in load_data_and_data_loaders
• Checks off the datasets TODO in the README

Each new dataset uses sensible defaults:

• StreetFighterDataset: 128x128, 15fps (rich visual detail, fast-paced action)
• TerrariaDataset: 128x128, 15fps (side-scroller similar to Zelda/Sonic)
• SpaceInvadersDataset: 64x64, 15fps, 2x frame subsampling (simpler visuals, slower motion)

To use, place your mp4 at the corresponding /data/ path (e.g. street_fighter.mp4) and pass the dataset name string to the config.

Test plan

• Drop an mp4 into /data/street_fighter.mp4 and run python scripts/full_train.py --config configs/training.yaml -- dataset=STREET_FIGHTER
• Same for terraria.mp4 / TERRARIA and space_invaders.mp4 / SPACE_INVADERS
• Verify dataset loads without error and produces valid frame sequences