`edges`

The shared edge-list core behind the network family (Mark.CHORD/ARC_DIAGRAM/GRAPH/SANKEY). All four read the same three columns from Plot.encode_chord() (a from name, a to name, and a magnitude per row) and need the same two things before any mark-specific layout: validation of those columns, and each endpoint’s position in the node domain.

Same shape as hierarchy.mojo for the hierarchy family: the family’s shared data type plus the shared index it resolves into, in one module the family’s marks import. _EdgeData lives here rather than on Plot because four marks read it.