fix: synthesise Contains edges and improve cross-file resolution in code graph

The code graph produced disconnected "islands" because:
1. No Contains edges were created between File/Module nodes and their
   children (functions, classes, structs), leaving file nodes isolated
2. Cross-file call resolution was too strict — calls like
   `crate::config::load` failed to resolve to `src/config.rs::load`

Fix:
- After resolving explicit parser edges, synthesise Contains edges by
  walking each node's qualified-name hierarchy and linking to the
  closest ancestor that exists in the node map
- Improve edge resolution with module-path matching: strip Rust
  prefixes (crate::, super::, self::) and try progressively shorter
  suffix matches for cross-file calls

Adds 4 new tests covering Contains edge synthesis, dedup with existing
edges, cross-file module path resolution, and parent qname lookup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

compliance-graph/src/graph/engine.rs

@@ -15,6 +15,30 @@ use crate::parsers::registry::ParserRegistry;
 use super::community::detect_communities;
 use super::impact::ImpactAnalyzer;
 
+/// Walk up the qualified-name hierarchy to find the closest ancestor
+/// that exists in the node map.
+///
+/// For `"src/main.rs::config::load"` this tries:
+///   1. `"src/main.rs::config"`  (trim last `::` segment)
+///   2. `"src/main.rs"`          (trim again)
+///
+/// Returns the first match found, or `None` if the node is a root.
+fn find_parent_qname(qname: &str, node_map: &HashMap<String, NodeIndex>) -> Option<String> {
+    let mut current = qname.to_string();
+    loop {
+        // Try stripping the last "::" segment
+        if let Some(pos) = current.rfind("::") {
+            current.truncate(pos);
+            if node_map.contains_key(&current) {
+                return Some(current);
+            }
+            continue;
+        }
+        // No more "::" — this is a top-level node (file), no parent
+        return None;
+    }
+}
+
 /// The main graph engine that builds and manages code knowledge graphs
 pub struct GraphEngine {
     parser_registry: ParserRegistry,
@@ -89,7 +113,12 @@ impl GraphEngine {
         Ok((code_graph, build_run))
     }
 
-    /// Build petgraph from parsed output, resolving edges to node indices
+    /// Build petgraph from parsed output, resolving edges to node indices.
+    ///
+    /// After resolving the explicit edges from parsers, we synthesise
+    /// `Contains` edges so that every node is reachable from its parent
+    /// file or module.  This eliminates disconnected "islands" that
+    /// otherwise appear when files share no direct call/import edges.
     fn build_petgraph(&self, parse_output: ParseOutput) -> Result<CodeGraph, CoreError> {
         let mut graph = DiGraph::new();
         let mut node_map: HashMap<String, NodeIndex> = HashMap::new();
@@ -102,15 +131,13 @@ impl GraphEngine {
             node_map.insert(node.qualified_name.clone(), idx);
         }
 
-        // Resolve and add edges — rewrite target to the resolved qualified name
-        // so the persisted edge references match node qualified_names.
+        // Resolve and add explicit edges from parsers
         let mut resolved_edges = Vec::new();
         for mut edge in parse_output.edges {
             let source_idx = node_map.get(&edge.source);
             let resolved = self.resolve_edge_target(&edge.target, &node_map);
 
             if let (Some(&src), Some(tgt)) = (source_idx, resolved) {
-                // Update target to the resolved qualified name
                 let resolved_name = node_map
                     .iter()
                     .find(|(_, &idx)| idx == tgt)
@@ -121,7 +148,48 @@ impl GraphEngine {
                 graph.add_edge(src, tgt, edge.kind.clone());
                 resolved_edges.push(edge);
             }
-            // Skip unresolved edges (cross-file, external deps) — conservative approach
+        }
+
+        // Synthesise Contains edges: connect each node to its closest
+        // parent in the qualified-name hierarchy.
+        //
+        // For "src/main.rs::config::load", the parent chain is:
+        //   "src/main.rs::config" → "src/main.rs"
+        //
+        // We walk up the qualified name (splitting on "::") and link to
+        // the first ancestor that exists in the node map.
+        let repo_id = nodes.first().map(|n| n.repo_id.as_str()).unwrap_or("");
+        let build_id = nodes
+            .first()
+            .map(|n| n.graph_build_id.as_str())
+            .unwrap_or("");
+
+        let qualified_names: Vec<String> = nodes.iter().map(|n| n.qualified_name.clone()).collect();
+        let file_paths: HashMap<String, String> = nodes
+            .iter()
+            .map(|n| (n.qualified_name.clone(), n.file_path.clone()))
+            .collect();
+
+        for qname in &qualified_names {
+            if let Some(parent_qname) = find_parent_qname(qname, &node_map) {
+                let child_idx = node_map[qname];
+                let parent_idx = node_map[&parent_qname];
+
+                // Avoid duplicate edges
+                if !graph.contains_edge(parent_idx, child_idx) {
+                    graph.add_edge(parent_idx, child_idx, CodeEdgeKind::Contains);
+                    resolved_edges.push(CodeEdge {
+                        id: None,
+                        repo_id: repo_id.to_string(),
+                        graph_build_id: build_id.to_string(),
+                        source: parent_qname,
+                        target: qname.clone(),
+                        kind: CodeEdgeKind::Contains,
+                        file_path: file_paths.get(qname).cloned().unwrap_or_default(),
+                        line_number: None,
+                    });
+                }
+            }
         }
 
         Ok(CodeGraph {
@@ -132,33 +200,62 @@ impl GraphEngine {
         })
     }
 
-    /// Try to resolve an edge target to a known node
+    /// Try to resolve an edge target to a known node.
+    ///
+    /// Resolution strategies (in order):
+    /// 1. Direct qualified-name match
+    /// 2. Suffix match: "foo" matches "src/main.rs::mod::foo"
+    /// 3. Module-path match: "config::load" matches "src/config.rs::load"
+    /// 4. Self-method: "self.method" matches "::method"
     fn resolve_edge_target(
         &self,
         target: &str,
         node_map: &HashMap<String, NodeIndex>,
     ) -> Option<NodeIndex> {
-        // Direct match
+        // 1. Direct match
         if let Some(idx) = node_map.get(target) {
             return Some(*idx);
         }
 
-        // Try matching just the function/type name (intra-file resolution)
+        // 2. Suffix match: "foo" → "path/file.rs::foo"
+        let suffix_pattern = format!("::{target}");
+        let dot_pattern = format!(".{target}");
         for (qualified, idx) in node_map {
-            // Match "foo" to "path/file.rs::foo" or "path/file.rs::Type::foo"
-            if qualified.ends_with(&format!("::{target}"))
-                || qualified.ends_with(&format!(".{target}"))
-            {
+            if qualified.ends_with(&suffix_pattern) || qualified.ends_with(&dot_pattern) {
                 return Some(*idx);
             }
         }
 
-        // Try matching method calls like "self.method" -> look for "::method"
+        // 3. Module-path match: "config::load" → try matching the last N
+        //    segments of the target against node qualified names.
+        //    This handles cross-file calls like `crate::config::load` or
+        //    `super::handlers::process` where the prefix differs.
+        if target.contains("::") {
+            // Strip common Rust path prefixes
+            let stripped = target
+                .strip_prefix("crate::")
+                .or_else(|| target.strip_prefix("super::"))
+                .or_else(|| target.strip_prefix("self::"))
+                .unwrap_or(target);
+
+            let segments: Vec<&str> = stripped.split("::").collect();
+            // Try matching progressively shorter suffixes
+            for start in 0..segments.len() {
+                let suffix = segments[start..].join("::");
+                let pattern = format!("::{suffix}");
+                for (qualified, idx) in node_map {
+                    if qualified.ends_with(&pattern) {
+                        return Some(*idx);
+                    }
+                }
+            }
+        }
+
+        // 4. Self-method: "self.method" → "::method"
         if let Some(method_name) = target.strip_prefix("self.") {
+            let pattern = format!("::{method_name}");
             for (qualified, idx) in node_map {
-                if qualified.ends_with(&format!("::{method_name}"))
-                    || qualified.ends_with(&format!(".{method_name}"))
-                {
+                if qualified.ends_with(&pattern) {
                     return Some(*idx);
                 }
             }
@@ -353,4 +450,83 @@ mod tests {
         assert!(code_graph.node_map.contains_key("a::c"));
         assert!(code_graph.node_map.contains_key("a::d"));
     }
+
+    #[test]
+    fn test_contains_edges_synthesised() {
+        let engine = GraphEngine::new(1000);
+        let mut output = ParseOutput::default();
+        // File → Module → Function hierarchy
+        output.nodes.push(make_node("src/main.rs"));
+        output.nodes.push(make_node("src/main.rs::config"));
+        output.nodes.push(make_node("src/main.rs::config::load"));
+
+        let code_graph = engine.build_petgraph(output).unwrap();
+
+        // Should have 2 Contains edges:
+        //   src/main.rs → src/main.rs::config
+        //   src/main.rs::config → src/main.rs::config::load
+        let contains_edges: Vec<_> = code_graph
+            .edges
+            .iter()
+            .filter(|e| matches!(e.kind, CodeEdgeKind::Contains))
+            .collect();
+        assert_eq!(contains_edges.len(), 2, "expected 2 Contains edges");
+
+        let sources: Vec<&str> = contains_edges.iter().map(|e| e.source.as_str()).collect();
+        assert!(sources.contains(&"src/main.rs"));
+        assert!(sources.contains(&"src/main.rs::config"));
+    }
+
+    #[test]
+    fn test_contains_edges_no_duplicates_with_existing_edges() {
+        let engine = GraphEngine::new(1000);
+        let mut output = ParseOutput::default();
+        output.nodes.push(make_node("src/main.rs"));
+        output.nodes.push(make_node("src/main.rs::foo"));
+
+        // Explicit Calls edge (foo calls itself? just for testing)
+        output.edges.push(CodeEdge {
+            id: None,
+            repo_id: "test".to_string(),
+            graph_build_id: "build1".to_string(),
+            source: "src/main.rs::foo".to_string(),
+            target: "src/main.rs::foo".to_string(),
+            kind: CodeEdgeKind::Calls,
+            file_path: "src/main.rs".to_string(),
+            line_number: Some(1),
+        });
+
+        let code_graph = engine.build_petgraph(output).unwrap();
+
+        // 1 Calls + 1 Contains = 2 edges total
+        assert_eq!(code_graph.edges.len(), 2);
+    }
+
+    #[test]
+    fn test_cross_file_resolution_with_module_path() {
+        let engine = GraphEngine::new(1000);
+        let node_map = build_test_node_map(&["src/config.rs::load_config", "src/main.rs::main"]);
+        // "crate::config::load_config" should resolve to "src/config.rs::load_config"
+        let result = engine.resolve_edge_target("crate::config::load_config", &node_map);
+        assert!(result.is_some(), "cross-file crate:: path should resolve");
+    }
+
+    #[test]
+    fn test_find_parent_qname() {
+        let node_map = build_test_node_map(&[
+            "src/main.rs",
+            "src/main.rs::config",
+            "src/main.rs::config::load",
+        ]);
+
+        assert_eq!(
+            find_parent_qname("src/main.rs::config::load", &node_map),
+            Some("src/main.rs::config".to_string())
+        );
+        assert_eq!(
+            find_parent_qname("src/main.rs::config", &node_map),
+            Some("src/main.rs".to_string())
+        );
+        assert_eq!(find_parent_qname("src/main.rs", &node_map), None);
+    }
 }