the joined-results from getEdgesLine() and getFacesPlane()
for every edge, for each of its 1 or 2 adjacent faces, a +1 or -1 for each face indicating which side of the edge the face lies on.
For every edge in a 2D graph, for each of its faces, get which side of the edge this face lies along. "Sidedness" is simply local to each edge, similar or collinear edges will not have a consistent side between them.
a FOLD object where, if faces_center exists then only edges_faces is needed, otherwise vertices and face data is needed.