Bump to 0.2.1; derive __version__ from package metadata

0.2.0 shipped with a hardcoded __version__ = "0.1.0" that nothing kept in
sync with pyproject.toml. Read the version from importlib.metadata so the
package and module versions can never drift again.

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

Bump version to 0.2.0

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