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.
