Builds a map from NodeKey → [x, z] world position for every lane endpoint
in the junction. Uses the road-side ("end") point for in-lanes and the
junction-side ("start") point for out-lanes so that spawn/despawn nodes
sit at the road tips and internal nodes sit near the junction centre.
Builds a map from NodeKey → [x, z] world position for every lane endpoint in the junction. Uses the road-side ("end") point for in-lanes and the junction-side ("start") point for out-lanes so that spawn/despawn nodes sit at the road tips and internal nodes sit near the junction centre.