feat(graph): vault link-graph model — notes to nodes, wikilinks to edges (T-323)

VaultGraph.fromOutlinks builds the graph from pql's per-file outlinks: every
file a node (labelled by basename), each outlink to a known file an edge;
self/dangling links dropped, parallels de-duped. neighborhood() drives the
hover-highlight; edgePairs feeds the ForceLayout solver. Pure Dart,
dart-tested (6 cases). The graph pane's data layer, on top of the solver.

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

test/src/graph/vault_graph_test.dart

@@ -0,0 +1,51 @@
+import 'package:clide/src/graph/vault_graph.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('VaultGraph.fromOutlinks', () {
+    test('every file is a node; its label is the basename without extension', () {
+      final g = VaultGraph.fromOutlinks({'notes/alpha.md': const [], 'beta.md': const []});
+      expect(g.nodes.map((n) => n.id), containsAll(['notes/alpha.md', 'beta.md']));
+      expect(g.nodes.firstWhere((n) => n.id == 'notes/alpha.md').label, 'alpha');
+    });
+
+    test('outlinks to known files become edges', () {
+      final g = VaultGraph.fromOutlinks({
+        'a.md': const ['b.md'],
+        'b.md': const [],
+      });
+      expect(g.edgePairs, [('a.md', 'b.md')]);
+    });
+
+    test('self-links and dangling links are dropped', () {
+      final g = VaultGraph.fromOutlinks({
+        'a.md': const ['a.md', 'ghost.md', 'b.md'],
+        'b.md': const [],
+      });
+      expect(g.edgePairs, [('a.md', 'b.md')]); // no self, no ghost
+    });
+
+    test('parallel edges are de-duplicated', () {
+      final g = VaultGraph.fromOutlinks({
+        'a.md': const ['b.md', 'b.md'],
+        'b.md': const [],
+      });
+      expect(g.edgePairs, hasLength(1));
+    });
+
+    test('an empty vault is empty', () {
+      expect(VaultGraph.fromOutlinks(const {}).isEmpty, isTrue);
+    });
+  });
+
+  test('neighborhood returns a node plus its direct links, both directions', () {
+    final g = VaultGraph.fromOutlinks({
+      'a.md': const ['b.md'],
+      'b.md': const ['c.md'],
+      'c.md': const [],
+      'x.md': const [],
+    });
+    expect(g.neighborhood('b.md'), {'b.md', 'a.md', 'c.md'});
+    expect(g.neighborhood('x.md'), {'x.md'}); // isolated node
+  });
+}