feat(graph): pan + zoom on the vault graph view (T-323)

Folds a user zoom (scroll wheel, clamped) and pan (drag) transform into
GraphViewport so the painter and hit-testing move in lockstep — hover and
click keep landing on what's drawn. Zoom scales about the canvas centre; a
fresh graph re-fits and drops the transform.

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

test/builtin/graph/graph_painter_test.dart

@@ -69,6 +69,23 @@ void main() {
     expect(await tester.runAsync(() => hasInk(GraphPainter(graph: const VaultGraph([], []), positions: const {}, tokens: tokens))), isFalse);
   });
 
+  testWidgets('paints under a user zoom + pan', (tester) async {
+    final tokens = await tokensFrom(tester);
+    final g = twoNodes();
+    final pos = ForceLayout.compute(['a.md', 'b.md'], g.edgePairs);
+    expect(await tester.runAsync(() => hasInk(GraphPainter(graph: g, positions: pos, tokens: tokens, zoom: 2, pan: const Offset(15, -10)))), isTrue);
+  });
+
+  testWidgets('repaints when zoom or pan changes, not when identical', (tester) async {
+    final tokens = await tokensFrom(tester);
+    final g = twoNodes();
+    final pos = ForceLayout.compute(['a.md', 'b.md'], g.edgePairs);
+    GraphPainter p({double zoom = 1, Offset pan = Offset.zero}) => GraphPainter(graph: g, positions: pos, tokens: tokens, zoom: zoom, pan: pan);
+    expect(p().shouldRepaint(p()), isFalse);
+    expect(p(zoom: 2).shouldRepaint(p()), isTrue);
+    expect(p(pan: const Offset(1, 0)).shouldRepaint(p()), isTrue);
+  });
+
   group('hitTestNode', () {
     test('finds the node under the point, null when far or empty', () {
       final g = VaultGraph.fromOutlinks({'a.md': const []});
@@ -79,5 +96,25 @@ void main() {
       expect(hitTestNode(g, pos, const Offset(20, 20), const Size(800, 600)), isNull);
       expect(hitTestNode(g, const {}, Offset.zero, const Size(800, 600)), isNull);
     });
+
+    test('pan shifts the hit location; zoom scales about the centre', () {
+      final g = VaultGraph.fromOutlinks({'a.md': const []});
+      final pos = ForceLayout.compute(['a.md'], const []); // single node at (400,300)
+      // A pan moves the node by the same pixels — hit follows it, misses the old spot.
+      expect(hitTestNode(g, pos, const Offset(500, 300), const Size(800, 600), pan: const Offset(100, 0)), 'a.md');
+      expect(hitTestNode(g, pos, const Offset(400, 300), const Size(800, 600), pan: const Offset(100, 0)), isNull);
+      // Zoom scales about the canvas centre, so a centre node stays under it.
+      expect(hitTestNode(g, pos, const Offset(400, 300), const Size(800, 600), zoom: 3), 'a.md');
+    });
+  });
+
+  group('GraphViewport.fit', () {
+    test('a centre point is pan-translated and zoom-invariant', () {
+      const canvas = Size(800, 600), layout = Size(800, 600);
+      final centre = (x: 400.0, y: 300.0);
+      expect(GraphViewport.fit(canvas, layout).toPixel(centre), const Offset(400, 300));
+      expect(GraphViewport.fit(canvas, layout, zoom: 4).toPixel(centre), const Offset(400, 300));
+      expect(GraphViewport.fit(canvas, layout, pan: const Offset(10, 20)).toPixel(centre), const Offset(410, 320));
+    });
   });
 }

test/builtin/graph/graph_view_test.dart

@@ -1,5 +1,6 @@
 import 'package:clide/builtin/graph/src/graph_view.dart';
 import 'package:clide/src/graph/vault_graph.dart';
+import 'package:flutter/gestures.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -36,4 +37,47 @@ void main() {
     expect(find.byType(GraphView), findsOneWidget);
     await tester.tap(find.byType(GraphView)); // no node, no onOpen — must not throw
   });
+
+  testWidgets('dragging pans the graph; a reverse pan restores it', (tester) async {
+    String? opened;
+    final g = VaultGraph.fromOutlinks({'only.md': const []});
+    await tester.pumpWidget(
+      anchoredHarness(
+        f,
+        SizedBox(
+          width: 400,
+          height: 400,
+          child: GraphView(graph: g, onOpen: (id) => opened = id),
+        ),
+      ),
+    );
+    await tester.pump();
+    final centre = tester.getCenter(find.byType(GraphView));
+    // Pan right — the node leaves the centre, so a centre tap misses. (An equal,
+    // opposite drag cancels the same gesture slop, so the net returns to zero —
+    // robust to the exact slop the arena consumes.)
+    await tester.drag(find.byType(GraphView), const Offset(120, 0));
+    await tester.pump();
+    await tester.tapAt(centre);
+    expect(opened, isNull);
+    await tester.drag(find.byType(GraphView), const Offset(-120, 0));
+    await tester.pump();
+    await tester.tapAt(centre);
+    expect(opened, 'only.md');
+  });
+
+  testWidgets('a scroll signal zooms without throwing', (tester) async {
+    final g = VaultGraph.fromOutlinks({
+      'a.md': const ['b.md'],
+      'b.md': const [],
+    });
+    await tester.pumpWidget(anchoredHarness(f, SizedBox(width: 400, height: 400, child: GraphView(graph: g))));
+    await tester.pump();
+    final centre = tester.getCenter(find.byType(GraphView));
+    final pointer = TestPointer(1, PointerDeviceKind.mouse);
+    await tester.sendEventToBinding(pointer.hover(centre));
+    await tester.sendEventToBinding(pointer.scroll(const Offset(0, -120))); // zoom in
+    await tester.pump();
+    expect(find.byType(GraphView), findsOneWidget);
+  });
 }