Introduce nested batching scopes to automate component dirty-marking
and defer structural mutations.
- Replace manual `MarkModified` with auto-tracking via `RefN` properties
in `Select` iterators and `GetComponent`/`GetSingleton` calls.
- Implement nested batching scopes: `SystemGroup` (tick level),
`ISystem.Run`
(system level), `CommandQueue.ExecuteAll` (command level), and
`Select`
(iteration level).
- Update `ISystem` and `ITickedSystem` to use `RunImpl` to separate
implementation from the batching-aware `Run` extension method.
- Add `ValN` properties to iterators for read-only, non-tracked access.
- Update documentation to reflect the new iteration and batching model.
Introduce `TrackAccess` in `World` to record component access
during batching scopes. Update `SelectN` iterators to provide
`RefN` for mutable, tracked access and `ValN` for read-only,
untracked access.
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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