sceneFixedPass method
Walks this node's subtree once per physics substep and dispatches
Component.fixedTick to every component.
Called by Scene's fixed-step driver inside its substepping loop, before the physics world's step. Traversal order is parent before children, matching scenePrePass.
Implementation
void sceneFixedPass(double fixedDt) {
_visitMutable(_components, (component) => component.fixedTick(fixedDt));
_visitMutable(children, (child) => child.sceneFixedPass(fixedDt));
}