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.
hypercross
4b94f411bf