feat: 双链 link graph pure core — [[link]] parse + backlinks (F7)

Pure bidirectional-link engine for the sticky-note/board system (a user-named
differentiator). parseLinkTargets extracts trimmed, de-duped [[targets]];
LinkGraph builds forward + backlink indices (fromTexts parses, fromLinks takes
explicit targets), ignores self-links, and danglingTargets() surfaces links to
unknown nodes. Widget-free + storage-free so it is fully unit-tested; the board
UI + persistence wrap it later.

Built ahead of its phase deliberately as a zero-rework-risk pure data structure
(not rendering/perf — the P0.5 device gate can't invalidate it).

flutter analyze lib/editor clean; 139/139 tests (+10: parsing, backlinks,
self-link, dangling, immutability).

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

test/link_graph_test.dart

@@ -0,0 +1,81 @@
+// Tests for the 双链 (bidirectional link) pure core (F7): [[link]] parsing +
+// forward/backlink indices + dangling-link detection.
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/link/link_graph.dart';
+
+void main() {
+  group('parseLinkTargets', () {
+    test('extracts bracketed targets, trimmed', () {
+      expect(parseLinkTargets('see [[Alpha]] and [[ Beta ]]'),
+          ['Alpha', 'Beta']);
+    });
+
+    test('de-duplicates, keeping first-occurrence order', () {
+      expect(parseLinkTargets('[[A]] [[B]] [[A]]'), ['A', 'B']);
+    });
+
+    test('ignores empty / whitespace-only targets', () {
+      expect(parseLinkTargets('x [[]] y [[   ]] z [[Real]]'), ['Real']);
+    });
+
+    test('no links → empty', () {
+      expect(parseLinkTargets('plain text, no links'), isEmpty);
+    });
+  });
+
+  group('LinkGraph', () {
+    test('backlinksOf collects every node linking to a target', () {
+      final g = LinkGraph.fromTexts({
+        'n1': 'links to [[n2]] and [[n3]]',
+        'n2': 'links to [[n3]]',
+        'n3': 'a leaf',
+      });
+      expect(g.linksFrom('n1'), {'n2', 'n3'});
+      expect(g.linksFrom('n3'), isEmpty);
+      expect(g.backlinksOf('n3'), {'n1', 'n2'});
+      expect(g.backlinksOf('n2'), {'n1'});
+      expect(g.backlinksOf('n1'), isEmpty);
+    });
+
+    test('self-links are ignored', () {
+      final g = LinkGraph.fromTexts({'n1': 'I reference [[n1]] myself'});
+      expect(g.linksFrom('n1'), isEmpty);
+      expect(g.backlinksOf('n1'), isEmpty);
+    });
+
+    test('fromLinks builds the same indices from explicit targets', () {
+      final g = LinkGraph.fromLinks({
+        'a': ['b', 'c'],
+        'b': ['c'],
+        'c': [],
+      });
+      expect(g.backlinksOf('c'), {'a', 'b'});
+      expect(g.linksFrom('a'), {'b', 'c'});
+    });
+
+    test('danglingTargets surfaces links to unknown nodes', () {
+      final g = LinkGraph.fromLinks({
+        'a': ['b', 'ghost'],
+        'b': [],
+      });
+      expect(g.danglingTargets(), {'ghost'});
+    });
+
+    test('returned sets are unmodifiable', () {
+      final g = LinkGraph.fromLinks({
+        'a': ['b'],
+        'b': [],
+      });
+      expect(() => g.linksFrom('a').add('x'), throwsUnsupportedError);
+      expect(() => g.backlinksOf('b').add('x'), throwsUnsupportedError);
+    });
+
+    test('unknown node ids yield empty (no crash)', () {
+      final g = LinkGraph.fromLinks({'a': []});
+      expect(g.linksFrom('nope'), isEmpty);
+      expect(g.backlinksOf('nope'), isEmpty);
+    });
+  });
+}