We don't really have a good end-to-end demo at this point. We're still building out some of the "core" pieces of state management (namely the Scene system, which works in the current form but isn't fully featured yet). In general transitions between menus/levels will happen via Scene transitions (similar to how godot handles it).

You can also adopt a `enum State { Menu, InGame }` Resource pattern, then have your systems key off of that state where necessary.