OptionaledgesRouted edge polylines, keyed by edge id. Present when the layout ran
with routeEdges (default); a renderer follows these to route around
nodes. Synthetic chain edges are never included. Optional so an
endpoint-only consumer can ignore it.
Header strip height used for every container (renderers reserve it).
Versioned geometry document (the third integration channel: versioned JSON documents). Renderer-neutral: absolute top-left boxes; a Cytoscape consumer converts to center positions.