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>

lib/editor/board/board.dart

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+// lib/editor/board/board.dart
+//
+// Infinite-board model (F7 — 便利贴 + 双链). A board is a set of positioned
+// cards (sticky notes) in board content coordinates; each card has text that may
+// contain [[links]] to other cards, so a board derives a LinkGraph for
+// backlinks. Cards also host ink (via a StrokeHost keyed by the card id) — the
+// same host-agnostic engine as PDF pages.
+//
+// Pure, immutable value model (no widgets/storage); fully unit-tested. The board
+// canvas + persistence wrap it.
+
+import 'dart:ui' show Offset, Rect, Size;
+
+import 'package:flutter/foundation.dart';
+
+import '../link/link_graph.dart';
+
+/// One sticky-note card on the board.
+@immutable
+class BoardCard {
+  const BoardCard({
+    required this.id,
+    required this.position,
+    required this.size,
+    this.text = '',
+  });
+
+  final String id;
+
+  /// Top-left in board content coordinates.
+  final Offset position;
+  final Size size;
+
+  /// Card body; may contain `[[other-card]]` links.
+  final String text;
+
+  Rect get bounds => position & size;
+
+  BoardCard copyWith({Offset? position, Size? size, String? text}) => BoardCard(
+        id: id,
+        position: position ?? this.position,
+        size: size ?? this.size,
+        text: text ?? this.text,
+      );
+
+  @override
+  bool operator ==(Object other) =>
+      other is BoardCard &&
+      other.id == id &&
+      other.position == position &&
+      other.size == size &&
+      other.text == text;
+
+  @override
+  int get hashCode => Object.hash(id, position, size, text);
+}
+
+/// An immutable infinite board: an ordered list of cards with copy-on-write
+/// edits. Card ids are unique.
+@immutable
+class Board {
+  Board(List<BoardCard> cards) : cards = List<BoardCard>.unmodifiable(cards);
+
+  static final Board empty = Board(const []);
+
+  final List<BoardCard> cards;
+
+  int get length => cards.length;
+
+  BoardCard? cardById(String id) {
+    for (final c in cards) {
+      if (c.id == id) return c;
+    }
+    return null;
+  }
+
+  /// Add a card (throws if [card].id already exists).
+  Board add(BoardCard card) {
+    if (cardById(card.id) != null) {
+      throw ArgumentError('duplicate card id: ${card.id}');
+    }
+    return Board([...cards, card]);
+  }
+
+  Board removeById(String id) =>
+      Board([for (final c in cards) if (c.id != id) c]);
+
+  /// Replace card [id] via [update]; no-op if absent.
+  Board updateCard(String id, BoardCard Function(BoardCard) update) =>
+      Board([for (final c in cards) if (c.id == id) update(c) else c]);
+
+  Board moveCard(String id, Offset position) =>
+      updateCard(id, (c) => c.copyWith(position: position));
+
+  Board setText(String id, String text) =>
+      updateCard(id, (c) => c.copyWith(text: text));
+
+  /// Cards whose bounds overlap [viewport] (board broad-phase culling).
+  List<BoardCard> cardsIn(Rect viewport) =>
+      [for (final c in cards) if (c.bounds.overlaps(viewport)) c];
+
+  /// Derive the 双链 graph from card texts (card id → its [[links]]).
+  LinkGraph linkGraph() =>
+      LinkGraph.fromTexts({for (final c in cards) c.id: c.text});
+
+  /// Backlinks to card [id] (ids of cards whose text links to it).
+  Set<String> backlinksOf(String id) => linkGraph().backlinksOf(id);
+
+  @override
+  bool operator ==(Object other) =>
+      other is Board && listEquals(other.cards, cards);
+
+  @override
+  int get hashCode => Object.hashAll(cards);
+}