Function write_geometry 

fn write_geometry(
    geometry: &mut RouteGeometry,
    ends: Endpoints,
    path: &[GridPoint],
    locked: &HashSet<GridPoint>,
) -> Vec<Waypoint>

Overwrite the solved geometry from the canonical corner polyline path ([start, corners…, end]) and return the waypoint list that polyline promotes: interior corners in path order, locked iff they coincide with a previously locked corner. The list is the caller’s to push as an op — the solver writes geometry, never the document.