chore: add PyPI badge and Python 3.13/3.14 support

[dyson-025]

Closes #37

Summary

• Added a PyPI version badge to the README.
• Added Python 3.13 and 3.14 classifiers to pyproject.toml.
• Extended the CI matrix to include Python 3.13 and 3.14 so the advertised supported versions are covered by CI.

Testing

• No application logic changed.
• Verified documentation and metadata updates.

[higagan]

Verified locally and via CI — clean, correct, complete.

• CI is green on all five versions, including 3.13 and 3.14 (the full test suite, not just import), so the classifiers are backed by real coverage.
• The pypi/v badge URL resolves (HTTP 200), and the 3.13 / 3.14 classifiers land in the built package metadata.
• Matrix YAML is valid; ruff and the 96 tests pass.

Nicely scoped. Merging.