Graph DOT / JSON interchange

Suite path: conformance/interchange/graph_dot_json

fixture: parse the pinned DOT and JSON documents and confirm the same DAG (node_count, directed edges) is recovered. GML and NetworkX interchange also ship in antecedent-io (separate fixtures).

Expected summary

Top-level keys: description, dot, expected_edges, expected_node_count, fixture, json (6 fields).