Implement deferred structural mutations (AddComponent, RemoveComponent,
DestroyEntity) to allow safe modification of the world during
iteration.
Additionally, introduce a new `EntityIterator` API providing zero-
allocation `ref struct` iterators (`Select<T1>`, `Select<T1, T2>`, etc.)
to support manual iteration with early-exit capabilities.
As part of these changes, component access via `GetComponent<T>` during
iteration now automatically marks components as modified.
