Function spawn_physics_world_with 

pub fn spawn_physics_world_with<R: WorldRunner>(
    host: &mut World,
    config: PhysicsConfig,
) -> WorldHandle

Spawn the physics world driven by R: on its own thread (Threaded), or stepped by the host (Stepped).

The world owns the rapier world as a PhysicsWorld component, and gameplay never touches it: jobs build bodies, and a subscriber reads the state back. The world is created before the first job runs, so a job that builds a body — the static geometry, usually — finds it ready.

Nothing is published until someone subscribes: the world runs a subscriber’s collector once per step, after the step has drained its contacts. Subscribing with the handle’s JobSender is how a host asks for transforms, contacts, or both.

Both runners share one tick body, so a stepped world simulates exactly what a threaded one does: the only difference is that its host calls crate::world::step_stepped_world once per frame instead of a thread pacing itself.