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.
Introduces a complete query system for iterating over entities with
specific component combinations, including support for "with" and
"without" filters.
Also adds:
- `ISystem` and `ITickedSystem` interfaces for defining logic.
- `SystemGroup` for managing and running groups of systems.
- `QueryBuilder` for a fluent API to construct queries.
- `QueryExecutor` for high-performance iteration using ref parameters.
- `Tick` and `TickType` to support timed and logical updates.
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