chore: bump to v0.1.2, require Python 3.13

- version: 0.1.1 → 0.1.2 - requires-python: >=3.12 → >=3.13 (venv already runs 3.13.2; all 1716 tests pass; mypy clean on 3.13 target) - mypy python_version: 3.12 → 3.13 - README and docs/README version strings updated to match

Python 3.13 benefits relevant to this codebase: - 5–15% interpreter speedup - Better error messages (more specific, with suggestions) - copy.replace() for frozen dataclasses (MergeState, EditStep, TreeNode) - TypeVar defaults (PEP 696) for future generic work