perf(scan): index import edges once instead of scanning per file

imported_paths found the file node with a linear scan of all nodes and
then filtered all edges, for every Python/Dart file it built a set for:
O(files x (nodes + edges)) during resolution, about 10^10 comparisons
for 10k files and a million edges.

Imports edges are now indexed by source file on first use and the file
node is looked up by its stable id, as resolve_import already does. The
index is complete when first built: resolve_relationships resolves
every import before any call. Output is unchanged (all 124 tests pass).

src/scan.rs

@@ -248,6 +248,9 @@ struct ScanState {
     references: HashMap<String, HashSet<String>>,
     /// Lexical-language file -> repository files it imports (resolved edges).
     imported_paths: HashMap<String, Rc<HashSet<String>>>,
+    /// Imports edges by source file id, built once on first use: scanning every
+    /// node and edge per file made resolution O(files x (nodes + edges)).
+    imports_by_file: Option<HashMap<String, Vec<String>>>,
     /// `name:` of the root pubspec.yaml, so `package:<name>/x.dart` maps to lib/x.dart.
     dart_package: Option<String>,
 }
@@ -283,6 +286,7 @@ impl ScanState {
             rust: RustIndex::default(),
             references: HashMap::new(),
             imported_paths: HashMap::new(),
+            imports_by_file: None,
         }
     }
 
@@ -1319,20 +1323,27 @@ impl ScanState {
         if let Some(found) = self.imported_paths.get(path) {
             return Rc::clone(found);
         }
+        if self.imports_by_file.is_none() {
+            let mut index: HashMap<String, Vec<String>> = HashMap::new();
+            for edge in self.edges.iter().filter(|e| e.kind == EdgeKind::Imports) {
+                index
+                    .entry(edge.source.clone())
+                    .or_default()
+                    .push(edge.target.clone());
+            }
+            self.imports_by_file = Some(index);
+        }
+        let file_id = stable_id("file", path);
         let paths: HashSet<String> = self
-            .nodes
-            .iter()
-            .find(|n| n.kind == NodeKind::File && n.path.as_deref() == Some(path))
-            .map(|file| {
-                self.edges
-                    .iter()
-                    .filter(|e| e.kind == EdgeKind::Imports && e.source == file.id)
-                    .filter_map(|e| self.node(&e.target))
-                    .filter(|n| n.kind == NodeKind::File)
-                    .filter_map(|n| n.path.clone())
-                    .collect()
-            })
-            .unwrap_or_default();
+            .imports_by_file
+            .as_ref()
+            .and_then(|index| index.get(&file_id))
+            .into_iter()
+            .flatten()
+            .filter_map(|target| self.node(target))
+            .filter(|n| n.kind == NodeKind::File)
+            .filter_map(|n| n.path.clone())
+            .collect();
         let paths = Rc::new(paths);
         self.imported_paths
             .insert(path.to_owned(), Rc::clone(&paths));