perf(app): Phase 2 前端性能优化 5 项 — 8b-D01/D02/D03/M02/N01

- 8b-D01: Isar 添加 authorId+dateEpoch 复合索引和 dateEpoch 单独索引
- 8b-D02: getJournals 分页改为 DB 层 .offset().limit() 替代 Dart 层 sublist
- 8b-D03: home_bloc monthCount 改用日期范围独立查询（不受分页限制）
- 8b-M02: 笔画光栅化改为 BBox 裁剪 — 短笔画不再创建全画布尺寸图像
  - _CacheEntry 增加 offset 字段记录 BBox 偏移
  - _rasterizeStroke 计算包围盒 + 4px padding
  - _compositeIncremental 使用 offset 定位
- 8b-N01: SyncEngine enqueue 合并同一资源的操作
  - create+update → create（最新数据）
  - update+update → update（最新数据）
  - update+delete → delete
  - create+delete → 取消（不发送）
- 注意: Isar .g.dart 需运行 build_runner 重新生成

app/lib/features/editor/widgets/stroke_cache.dart

@@ -20,8 +20,10 @@ import 'stroke_renderer.dart';
 class _CacheEntry {
   final ui.Image image;
   final Stroke stroke;
+  /// BBox 偏移量 — 光栅化时裁剪的起点，合成时用于定位
+  final Offset offset;
 
-  const _CacheEntry({required this.image, required this.stroke});
+  const _CacheEntry({required this.image, required this.stroke, required this.offset});
 }
 
 // ===== 光栅化缓存 =====
@@ -76,18 +78,22 @@ class StrokeRasterCache {
 
   /// 添加一条已完成笔画到缓存
   ///
-  /// 光栅化该笔画为 ui.Image，然后增量合成到 compositeImage。
+  /// 光栅化该笔画为 ui.Image（仅 BBox 区域），然后增量合成到 compositeImage。
   Future<void> addStroke(Stroke stroke) async {
     if (_canvasSize == Size.zero) return;
 
-    // 光栅化单笔画
-    final image = await _rasterizeStroke(stroke);
-    if (image == null) return;
+    // 光栅化单笔画（BBox 裁剪）
+    final result = await _rasterizeStroke(stroke);
+    if (result == null) return;
 
-    _cache[stroke.id] = _CacheEntry(image: image, stroke: stroke);
+    _cache[stroke.id] = _CacheEntry(
+      image: result.image,
+      stroke: stroke,
+      offset: result.offset,
+    );
 
     // 增量合成：将新笔画绘制到现有 compositeImage 之上
-    await _compositeIncremental(stroke, image);
+    await _compositeIncremental(stroke, result.image, result.offset);
   }
 
   /// 同步笔画列表（用于撤销/重做后与 BLoC 状态对齐）
@@ -108,9 +114,13 @@ class StrokeRasterCache {
     final toAdd = currentIds.difference(cachedIds);
     for (final stroke in strokes) {
       if (toAdd.contains(stroke.id)) {
-        final image = await _rasterizeStroke(stroke);
-        if (image != null) {
-          _cache[stroke.id] = _CacheEntry(image: image, stroke: stroke);
+        final result = await _rasterizeStroke(stroke);
+        if (result != null) {
+          _cache[stroke.id] = _CacheEntry(
+            image: result.image,
+            stroke: stroke,
+            offset: result.offset,
+          );
         }
       }
     }
@@ -155,8 +165,12 @@ class StrokeRasterCache {
 
   // ===== 光栅化 =====
 
-  /// 将单条笔画光栅化为 ui.Image
-  Future<ui.Image?> _rasterizeStroke(Stroke stroke) async {
+  /// 将单条笔画光栅化为 ui.Image — 仅光栅化 BBox 区域（性能优化 8b-M02）
+  ///
+  /// 计算笔画的包围盒 (bounding box)，仅对该区域光栅化，
+  /// 大幅减少 GPU 内存占用（短笔画从全画布 4096×4096 降到实际尺寸）。
+  /// 返回 null 表示笔画无有效点。
+  Future<({ui.Image image, Offset offset})?> _rasterizeStroke(Stroke stroke) async {
     final outlinePoints = pointsToOutline(
       stroke.points,
       stroke.brushType,
@@ -165,16 +179,38 @@ class StrokeRasterCache {
     );
     if (outlinePoints.isEmpty) return null;
 
+    // 计算笔画包围盒
+    double minX = double.infinity, minY = double.infinity;
+    double maxX = double.negativeInfinity, maxY = double.negativeInfinity;
+    for (final p in outlinePoints) {
+      if (p.dx < minX) minX = p.dx;
+      if (p.dy < minY) minY = p.dy;
+      if (p.dx > maxX) maxX = p.dx;
+      if (p.dy > maxY) maxY = p.dy;
+    }
+
+    // 添加边距（抗锯齿 + 笔触溢出）
+    const padding = 4.0;
+    final bboxLeft = (minX - padding).clamp(0.0, _canvasSize.width);
+    final bboxTop = (minY - padding).clamp(0.0, _canvasSize.height);
+    final bboxRight = (maxX + padding).clamp(0.0, _canvasSize.width);
+    final bboxBottom = (maxY + padding).clamp(0.0, _canvasSize.height);
+    final bboxWidth = (bboxRight - bboxLeft).clamp(1.0, 4096.0);
+    final bboxHeight = (bboxBottom - bboxTop).clamp(1.0, 4096.0);
+
     final path = buildStrokePath(outlinePoints);
     final paint = createPaintForStroke(stroke);
 
     final recorder = ui.PictureRecorder();
     final canvas = Canvas(recorder);
 
+    // 平移坐标系，使 BBox 左上角对齐 (0, 0)
+    canvas.translate(-bboxLeft, -bboxTop);
+
     // 橡皮擦需要 saveLayer 保护，避免穿透
     if (stroke.brushType == BrushType.eraser) {
       canvas.saveLayer(
-        Rect.fromLTWH(0, 0, _canvasSize.width, _canvasSize.height),
+        Rect.fromLTWH(0, 0, bboxWidth, bboxHeight),
         Paint(),
       );
     }
@@ -186,16 +222,20 @@ class StrokeRasterCache {
     }
 
     final picture = recorder.endRecording();
-    return picture.toImage(
-      _canvasSize.width.toInt().clamp(1, 4096),
-      _canvasSize.height.toInt().clamp(1, 4096),
+    final image = await picture.toImage(
+      bboxWidth.toInt().clamp(1, 4096),
+      bboxHeight.toInt().clamp(1, 4096),
     );
+
+    return (image: image, offset: Offset(bboxLeft, bboxTop));
   }
 
   // ===== 合成 =====
 
   /// 增量合成：将新笔画图像绘制到现有 compositeImage 上
-  Future<void> _compositeIncremental(Stroke stroke, ui.Image strokeImage) async {
+  ///
+  /// strokeImage 是 BBox 裁剪后的图像，offset 是其原始位置偏移。
+  Future<void> _compositeIncremental(Stroke stroke, ui.Image strokeImage, Offset offset) async {
     final recorder = ui.PictureRecorder();
     final canvas = Canvas(recorder);
 
@@ -210,15 +250,15 @@ class StrokeRasterCache {
       canvas.drawImage(_compositeImage!, Offset.zero, Paint());
     }
 
-    // 再绘制新笔画（橡皮擦用 dstOut）
+    // 再绘制新笔画（橡皮擦用 dstOut），使用 BBox offset 定位
     if (stroke.brushType == BrushType.eraser) {
       canvas.drawImage(
         strokeImage,
-        Offset.zero,
+        offset,
         Paint()..blendMode = BlendMode.dstOut,
       );
     } else {
-      canvas.drawImage(strokeImage, Offset.zero, Paint());
+      canvas.drawImage(strokeImage, offset, Paint());
     }
 
     canvas.restore();