Replace the old `QueryBuilder` and `ForEach` callback pattern with a
more
performant, zero-allocation `Select` API using `ref struct` iterators.
This change also updates the singleton implementation to use dedicated
entities per component type rather than a single reserved entity.
- Replace `QueryBuilder`/`QueryDescriptor` with generic `Query<T1..T6>`
- Replace `ForEach` callbacks with `world.Select<T1..T6>()` iterators
- Replace `EntityIterator` with `WorldQueryExtensions`
- Update singleton logic to allocate one entity per component type
- Add `FindEntity<T>` and `FindEntities<T>` lookup methods
Add new skill documentation files for developing OECS, testing games,
and writing games. Also update the API surface documentation to include
guidance on component and command definitions.