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.
