feat(f7): infinite-board model (便利贴 cards) + derived 双链 backlinks

Board + BoardCard: positioned sticky-note cards in board coordinates, immutable
copy-on-write edits (add/removeById/moveCard/setText, unique ids), cardsIn()
broad-phase culling, and linkGraph()/backlinksOf() that derive the 双链 graph
from the cards' [[links]] — making link_graph load-bearing. Cards also host ink
via a StrokeHost keyed by card id (same host-agnostic engine as PDF pages).

Pure model; fully unit-tested (CRUD, immutability, culling, backlinks).

flutter analyze lib/editor clean; 215/215 tests (+7).

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

test/board_test.dart

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+// Tests for the infinite-board model (F7 便利贴 + 双链).
+
+import 'dart:ui' show Offset, Rect, Size;
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/board/board.dart';
+
+BoardCard _card(String id, double x, double y, {String text = ''}) => BoardCard(
+      id: id,
+      position: Offset(x, y),
+      size: const Size(100, 80),
+      text: text,
+    );
+
+void main() {
+  test('add / cardById / removeById', () {
+    final b = Board.empty.add(_card('a', 0, 0)).add(_card('b', 200, 0));
+    expect(b.length, 2);
+    expect(b.cardById('a')!.position, const Offset(0, 0));
+    expect(b.removeById('a').cardById('a'), isNull);
+  });
+
+  test('duplicate id throws', () {
+    final b = Board.empty.add(_card('a', 0, 0));
+    expect(() => b.add(_card('a', 5, 5)), throwsArgumentError);
+  });
+
+  test('moveCard / setText are copy-on-write', () {
+    final b0 = Board.empty.add(_card('a', 0, 0));
+    final b1 = b0.moveCard('a', const Offset(50, 60)).setText('a', 'hi');
+    expect(b0.cardById('a')!.position, const Offset(0, 0)); // original intact
+    expect(b1.cardById('a')!.position, const Offset(50, 60));
+    expect(b1.cardById('a')!.text, 'hi');
+  });
+
+  test('cardsIn culls cards outside the viewport', () {
+    final b = Board.empty
+        .add(_card('near', 0, 0)) // bounds 0,0,100,80
+        .add(_card('far', 5000, 5000));
+    final visible = b.cardsIn(const Rect.fromLTWH(0, 0, 300, 300));
+    expect(visible.map((c) => c.id).toSet(), {'near'});
+  });
+
+  test('linkGraph derives backlinks from card [[links]]', () {
+    final b = Board.empty
+        .add(_card('a', 0, 0, text: 'see [[b]] and [[c]]'))
+        .add(_card('b', 200, 0, text: 'see [[c]]'))
+        .add(_card('c', 400, 0, text: 'leaf'));
+    expect(b.backlinksOf('c'), {'a', 'b'});
+    expect(b.backlinksOf('b'), {'a'});
+    expect(b.backlinksOf('a'), isEmpty);
+    expect(b.linkGraph().linksFrom('a'), {'b', 'c'});
+  });
+
+  test('value equality + immutable card list', () {
+    final a = Board.empty.add(_card('x', 1, 1));
+    final b = Board.empty.add(_card('x', 1, 1));
+    expect(a, b);
+    expect(() => a.cards.add(_card('y', 0, 0)), throwsUnsupportedError);
+  });
+
+  test('BoardCard.bounds + copyWith', () {
+    final c = _card('a', 10, 20);
+    expect(c.bounds, const Rect.fromLTWH(10, 20, 100, 80));
+    expect(c.copyWith(text: 'z').text, 'z');
+    expect(c.copyWith().position, const Offset(10, 20));
+  });
+}