Remove the mandatory Query property from the ISystem interface,
allowing systems to manage their own queries locally or via other
means. Update all implementing systems and tests to use local
query descriptors instead.
Introduces a mechanism to store and access singleton components on a
reserved entity. The singleton entity is automatically excluded from
all standard queries to prevent it from being processed like regular
entities.
- Added `SetSingleton`, `GetSingleton`, `HasSingleton`, and
`RemoveSingleton` to `World`.
- Added `RegisterSingleton` to `EntityAllocator` to support the
reserved entity.
- Updated `QueryExecutor` to skip the singleton entity during
iteration.
- Added unit tests for singleton functionality.
hypercross