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_view_test.dart

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+import 'package:clide/builtin/graph/src/graph_view.dart';
+import 'package:clide/src/graph/vault_graph.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../../helpers/kernel_fixture.dart';
+import '../../helpers/widget_harness.dart';
+
+void main() {
+  late KernelFixture f;
+  setUp(() async => f = await KernelFixture.create());
+  tearDown(() => f.dispose());
+
+  testWidgets('clicking a node opens it', (tester) async {
+    String? opened;
+    final g = VaultGraph.fromOutlinks({'only.md': const []});
+    await tester.pumpWidget(
+      anchoredHarness(
+        f,
+        SizedBox(
+          width: 300,
+          height: 300,
+          child: GraphView(graph: g, layoutSize: const Size(300, 300), onOpen: (id) => opened = id),
+        ),
+      ),
+    );
+    await tester.pump();
+    // A single node lays out at the centre → the view's centre.
+    await tester.tap(find.byType(GraphView));
+    expect(opened, 'only.md');
+  });
+
+  testWidgets('renders an empty graph without error', (tester) async {
+    await tester.pumpWidget(anchoredHarness(f, const SizedBox(width: 200, height: 200, child: GraphView(graph: VaultGraph([], [])))));
+    await tester.pump();
+    expect(find.byType(GraphView), findsOneWidget);
+    await tester.tap(find.byType(GraphView)); // no node, no onOpen — must not throw
+  });
+}