Inits that are logically singletons (e.g. actor
location or owner) should implement this interface
to avoid runtime inconsistencies.
Duplicate instances are rejected at init-time,
allowing simpler queries when they are used.
- When removing entries, ensure the dictionary entry is removed is the list is emptied, to prevent incorrect lookups later.
- Prevent NRE when calling GetOrDefault<T> where T is a value type and the default is returned.
- Use a single dictionary for simplicity and improved lookup performance (since there is only one dictionary to check).
- De-duplicate code in Get and GetOrDefault.
