refactor: extract module_edges() to CodeGraph domain — removes duplication from Mermaid and D2 renderers

crates/domain/tests/code_graph_tests.rs

@@ -299,3 +299,59 @@ fn graph_lists_unique_modules() {
     assert!(modules.iter().any(|m| m.as_str() == "Orders"));
     assert!(modules.iter().any(|m| m.as_str() == "Billing"));
 }
+
+#[test]
+fn module_edges_aggregates_type_level_relationships_into_module_pairs() {
+    let mut graph = CodeGraph::new();
+    graph.add_element(make_element("ServiceA", Some("App")));
+    graph.add_element(make_element("ServiceB", Some("App")));
+    graph.add_element(make_element("Order", Some("Domain")));
+    graph.add_element(make_element("Product", Some("Domain")));
+
+    graph.add_relationship(
+        Relationship::new("ServiceA", "Order", RelationshipKind::Composition).unwrap(),
+    );
+    graph.add_relationship(
+        Relationship::new("ServiceB", "Product", RelationshipKind::Composition).unwrap(),
+    );
+
+    let graph = graph.qualify();
+    let edges = graph.module_edges();
+
+    assert_eq!(edges.len(), 1, "should have one cross-module edge pair");
+    assert_eq!(edges[&("App".to_string(), "Domain".to_string())], 2);
+}
+
+#[test]
+fn module_edges_handles_direct_module_to_module_relationships() {
+    let mut graph = CodeGraph::new();
+    graph.add_element(make_element("Order", Some("Domain")));
+    graph.add_element(make_element("Service", Some("App")));
+    // Direct module-name edge (injected by merge_project_deps)
+    graph.add_relationship(
+        Relationship::new("App", "Domain", RelationshipKind::Composition).unwrap(),
+    );
+
+    let graph = graph.qualify();
+    let edges = graph.module_edges();
+
+    assert!(
+        edges.contains_key(&("App".to_string(), "Domain".to_string())),
+        "direct module edge should appear: {edges:?}"
+    );
+}
+
+#[test]
+fn module_edges_excludes_intra_module_relationships() {
+    let mut graph = CodeGraph::new();
+    graph.add_element(make_element("Service", Some("App")));
+    graph.add_element(make_element("Helper", Some("App")));
+    graph.add_relationship(
+        Relationship::new("Service", "Helper", RelationshipKind::Composition).unwrap(),
+    );
+
+    let graph = graph.qualify();
+    let edges = graph.module_edges();
+
+    assert!(edges.is_empty(), "intra-module relationships should not produce edges");
+}