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.
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.
- Move examples (Blackjack, TicTacToe) to the root directory
- Convert example projects from Console applications to Libraries
- Add Directory.Build.props for shared build settings
- Update solution file and project references to reflect new paths
hypercross