Add sdist build and publish to release workflow

[b-long]

Currently the release workflow builds and publishes only wheel artifacts. We should also build source distributions (sdist) via uv build --sdist and include them in the PyPI upload, so that users on platforms without a matching wheel can install from source. The twine check step added in #163 already validates both artifact types, so the infrastructure is in place. This would also align with PyPI best practices for open-source packages.