you might want to look at geometry_polgyon_edges_2024.0.py, it should answer all your questions.

The TLDR is: full-edge maps as you hint at in your posting do exist in Cinema via helper functions, but they are often also not very useful. Internally, Cinema is operating as any other DCC application with the concept of what is often called a "half-edge" data structure. So, when you have the two polygons A and B:

  a---b---e
  | A | B |
  d---c---f

Which share an edge over the vertices b and c, this is actually two edges and not one, due to how normals work. For both polygons to have a normal facing in the same direction, they must have opposite winding orders, which means that the edge b-c is actually two edges: b->c and c->b. So, A could for example index the edge as bc, and B as cb. The example goes into more detail with this.

Cheers,
Ferdinand