Return the model or undefined if not registered.
All registered models in registration order.
Register a model. Throws if a model with the same id is already registered (registration-time-only; no live re-binding).
Return the model or throw UnknownTraitModelError.
In-memory map of TraitModel by id. The runtime singleton lives at
traitModelRegistry; tests and isolated callers can construct their own vianew TraitModelRegistry().