Function edgesAfterConnect

Applies the cardinality and acyclicity rules to a proposed connection and returns the edge list the new edge should be added onto, or null to reject the connection.

• Output side: a source port holds at most one outgoing edge — drawing from an already-wired port rewires it, dropping the previous edge.
• Cycles: a connection whose target already reaches the source (over the edges left after the rewire) would close a loop and is rejected, as is a direct self-loop.

A missing source handle is the success port (loaded edges fold out into success in policyToEdges); target handles play no role since inputs accept any number of edges.