Rename `BeginIteration`/`EndIteration` to `BeginBatching`/`EndBatching`
to reflect that the mechanism now covers both entity iteration and
system execution.
- Update `ISystem` and `ITickedSystem` to use `RunImpl` for logic.
- Add `SystemExtensions.Run` to wrap system execution in a batching
scope.
- Update `SystemGroup` to wrap system execution in a batching scope.
- Update `WorldQueryExtensions` to use the new batching terminology.
Replace the delegate-based ForEach pattern with zero-allocation ref
struct Select iterators. This change moves iteration logic from
QueryExecutor to WorldQueryExtensions and introduces a new fluent
Query<T> API.
Other changes:
- Update singleton handling to use a dictionary instead of a reserved
ID.
- Add IsSingleton flag to ComponentDescriptor.
- Replace QueryDescriptor with type-safe Query<T> structs.
- Update tests to use the new Select API.
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