fix(graph): treat class instantiation as a caller/callee edge (#774) (#804)

`callers <Class>` returned "No callers found" (or only the importing
file) even when a class's constructor was called from many sites, and
the instantiation sites were invisible — the opposite of what "what
breaks if I change this class?" should answer.

The `instantiates` edges already existed in the graph, correctly
attributed to the constructing function; they were simply excluded from
the caller/callee traversal, which queried only calls/references/imports.
Constructing a class is calling its constructor, so add `instantiates`
to the edge-kind set in both getCallers and getCallees (kept symmetric so
they stay inverses and `trace` can cross the instantiation boundary,
function -> class -> its methods). impact already traversed all edge
kinds, so it was unaffected.

Query-layer only — existing indexes benefit on upgrade with no re-index.
Verified on a Python fixture: `callers Supervisor` now returns the
construction sites (main/work/test_it), and a new graph test asserts
main() <-> DerivedClass via the instantiation. Full suite green.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

__tests__/graph.test.ts

@@ -293,6 +293,25 @@ export { main };
 
       expect(Array.isArray(callees)).toBe(true);
     });
+
+    it('treats class instantiation as a caller/callee of the class (#774)', () => {
+      // main() does `new DerivedClass(10, 'test')`. Constructing a class is
+      // calling its constructor, so main is a caller of DerivedClass and
+      // DerivedClass is a callee of main. Before #774 the `instantiates` edge
+      // was excluded from the caller/callee traversal, so `callers <Class>`
+      // returned the importing file (or nothing) and missed every
+      // construction site.
+      const derived = cg.getNodesByKind('class').find((n) => n.name === 'DerivedClass');
+      const main = cg.getNodesByKind('function').find((n) => n.name === 'main');
+      expect(derived).toBeDefined();
+      expect(main).toBeDefined();
+
+      const callerNames = cg.getCallers(derived!.id).map((c) => c.node.name);
+      expect(callerNames).toContain('main');
+
+      const calleeNames = cg.getCallees(main!.id).map((c) => c.node.name);
+      expect(calleeNames).toContain('DerivedClass');
+    });
   });
 
   describe('getImpactRadius()', () => {