feat(graph): interactive GraphView — hover-highlight + click-to-open (T-323)

GraphView lays a VaultGraph out with the force solver, paints it, and wires
hover (light the hovered node's neighbourhood, dim the rest) + click (onOpen
with the node's vault path). A shared GraphViewport keeps hit-testing aligned
with paint. Solver + model + painter + interactive view now stand; the pql
link-data wiring, filter, and MultitabPane/slot registration are the
remaining "integrate into the app" work.

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

test/builtin/graph/graph_painter_test.dart

@@ -68,4 +68,16 @@ void main() {
     final tokens = await tokensFrom(tester);
     expect(await tester.runAsync(() => hasInk(GraphPainter(graph: const VaultGraph([], []), positions: const {}, tokens: tokens))), isFalse);
   });
+
+  group('hitTestNode', () {
+    test('finds the node under the point, null when far or empty', () {
+      final g = VaultGraph.fromOutlinks({'a.md': const []});
+      // A single node lays out at the layout centre (400,300); an 800×600 canvas
+      // over an 800×600 layout is scale 1, no offset → the node sits at (400,300).
+      final pos = ForceLayout.compute(['a.md'], const []);
+      expect(hitTestNode(g, pos, const Offset(400, 300), const Size(800, 600)), 'a.md');
+      expect(hitTestNode(g, pos, const Offset(20, 20), const Size(800, 600)), isNull);
+      expect(hitTestNode(g, const {}, Offset.zero, const Size(800, 600)), isNull);
+    });
+  });
 }