Use immutable mappings and other cleanups

[hmgaudecker]

Fixes #136

• Replace dict by MappingProxyType, use Mapping for annotations. Note: These changes require dags #67
• Remove the name field from Regime. We now just pass regimes={"alive": regime_a, "dead": regime_b}. In the initialisation of Model, we make sure its keys and the fields of regime_id_class line up. Then do everything based on a mapping of regime_names_to_ids. Gets rid of the awkward typing we had before.
• Remove if TYPE_CHECKING entirely -- too much developer time is being spent on this in Python 3.14 at essentially no runtime gain

[timmens]

Very nice actually! Thanks a lot 🎉

No showstoppers, the only thing I noticed is that the CLAUDE.md file seems to be outdated with regard to some of the API changes.