Convert static _types dictionaries to dynamic cached class properties to
enable proper plugin type inheritance and avoid mutating shared state.
Key changes:
- Replace static _types dicts with @cached_classproperty decorators
- Update cached_classproperty to support proper caching with class names
- Remove manual _types mutation in plugin loading/unloading
- Add pluginload event and cache clearing for proper plugin integration
- Fix test to trigger type checking during item creation
This ensures plugin types are properly inherited through the class
hierarchy and eliminates issues with shared mutable state between
test runs.
3be4a89aee