- Added 03_train_val_split.py to create deterministic train/validation splits from data.txt or fallback text
- Updated .gitignore to un-comment .vscode/ directory exclusion
- Changed data.txt pattern to *.txt for better file matching in gitignore
- Script handles UTF-8 text loading with newline normalization and writes train.txt/val.txt files
- Includes doctest examples and proper type hints
