Release atomref 0.2.1

[IvanChernyshov]

Summary

This PR merges all changes since the 0.1.4 release and prepares atomref 0.2.1.

• Add provenance-aware neutral proatomic electron-density profiles for H–Lr, with validated interpolation, unit conversion, metadata, and packaged data.
• Add approximate interatomic-surface position estimation using equal-density boundaries and promolecular-density minima, including diagnostic result metadata.
• Expand and strengthen the element, registry, radii, X–H, transfer-policy, and public discovery APIs.
• Add comprehensive documentation and executable notebooks for density profiles, IAS methods, policies, and dataset use.
• Strengthen typing, citation metadata, packaging validation, release checks, and CI coverage across Python 3.10–3.14.

The release candidate has undergone repeated independent review and local validation, including 320 passing tests, strict mypy checks, CFF validation, distribution builds, and installed-wheel checks.