Very cool library! I've just started using it and I'm pretty impressed so far.

One minor nitpick: using systems is a bit more ceremonious compared to the simple lambda syntax allowed by queries. Since in most structured code you will be using systems anyway, it would be nice to allow a simpler syntax to declare very short systems inline. Flecs.NET already allows this, for example: https://github.com/d-bucur/flecs-survivors/blob/935c87c7a7e507430e8cf4cef8f91c1566ea0c82/src/Physics.cs#L106-L110

For now I am using my own helpers to allow me to use this syntax:

var root = new SystemRoot(world) {
	// new LongSystem(),
	LambdaSystems.New((ref Position p, Entity e) => Console.WriteLine("Hello1")),
	LambdaSystems.New((ref Position p, Entity e) => Console.WriteLine("Hello2")),
};

And the helpers:

class LambdaSystems {
	static public LambdaSystem<T1> New<T1>(ForEachEntity<T1> lambda) where T1 : struct, IComponent {
		return new LambdaSystem<T1>(lambda);
	}
}

class LambdaSystem<T1>(ForEachEntity<T1> lambda) : QuerySystem<T1> where T1 : struct, IComponent {
	protected override void OnUpdate() {
		Query.ForEachEntity(lambda);
	}
}

It would be nice to have something like this by default in the library and maybe showing it off in the example.