A maturin-action matrix builds Linux (x86_64 + aarch64), macOS (x86_64 +
aarch64), and Windows wheels plus an sdist, and publishes them to PyPI when a
`v*` tag is pushed. The `python` feature only pulls pyo3 + smartcore, so the
wheel is light and cross-compiles cleanly; abi3-py39 means one wheel per
platform covers Python 3.9+. A manual (workflow_dispatch) run builds without
publishing. Needs a `PYPI_API_TOKEN` repo secret.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>