perf(discover): 剧场与新片页网格改为按行懒加载

双列网格原来是两个 Column 一次性建出所有卡片，翻几页后上百张卡连同封面都留在树上；
现在改成按行的 SliverList，只构建可见行，剧场页/新片页/搜索结果的滚动容器一并换成 sliver。
搜索结果、加载失败等状态同步改成 sliver，滚动位置与下拉刷新行为保持不变。

test/discover/home_all_dramas_grid_test.dart

@@ -384,7 +384,7 @@ void main() {
 
     expect(refresh.edgeOffset, 52);
     expect(
-      tester.widget<ListView>(_verticalFeed).physics,
+      tester.widget<CustomScrollView>(_verticalFeed).physics,
       AppPullToRefresh.physics,
     );
     await tester.drag(_verticalFeed, const Offset(0, 240));
@@ -396,7 +396,8 @@ void main() {
 }
 
 final Finder _verticalFeed = find.byWidgetPredicate(
-  (widget) => widget is ListView && widget.scrollDirection == Axis.vertical,
+  (widget) =>
+      widget is CustomScrollView && widget.scrollDirection == Axis.vertical,
 );
 
 Future<void> _pumpTheater(

test/discover/theater_pages_test.dart

@@ -8,6 +8,7 @@ import 'package:fruitshorts/core/data/models/home_feed.dart';
 import 'package:fruitshorts/core/theme/app_colors.dart';
 import 'package:fruitshorts/core/widgets/app_pull_to_refresh.dart';
 import 'package:fruitshorts/pages/discover/discover_view.dart';
+import 'package:fruitshorts/pages/discover/widgets/drama_grid.dart';
 import 'package:fruitshorts/core/data/repositories/favorite_repository.dart';
 import 'package:fruitshorts/core/data/repositories/playback_repository.dart';
 import 'package:fruitshorts/core/data/repositories/watch_history_repository.dart';
@@ -234,25 +235,26 @@ void main() {
     await tester.pumpAndSettle();
 
     final list = find.byKey(const Key('new-releases-list'));
-    expect(tester.widget<ListView>(list).physics, AppPullToRefresh.physics);
-    await tester.drag(list, const Offset(0, -400));
-    await tester.pump();
-    expect(_countKeys('home-drama-card-'), 20);
+    expect(
+      tester.widget<CustomScrollView>(list).physics,
+      AppPullToRefresh.physics,
+    );
 
+    // 网格懒加载后只保留可见行，页数直接看传给网格的剧目数。
+    List<Drama> gridDramas() => tester
+        .widget<SliverDramaGrid>(find.byKey(const Key('new-releases-all-grid')))
+        .dramas;
+    expect(gridDramas(), hasLength(20));
+
+    // jumpTo 不发滚动通知，滑到底后再拖一下才会触发加载。
     final position = _scrollPosition(tester, const Key('new-releases-list'));
     position.jumpTo(position.maxScrollExtent);
     await tester.pump();
-    expect(find.byKey(const Key('new-releases-load-more')), findsOneWidget);
-    await tester.pump(const Duration(milliseconds: 299));
-    expect(_countKeys('home-drama-card-'), 20);
-    await tester.pump(const Duration(milliseconds: 1));
+    await tester.drag(list, const Offset(0, -60));
     await tester.pump();
-    expect(_countKeys('home-drama-card-'), 40);
-
+    expect(find.byKey(const Key('new-releases-load-more')), findsOneWidget);
     await tester.pumpAndSettle();
-    position.jumpTo(position.maxScrollExtent);
-    await tester.pumpAndSettle();
-    expect(find.byKey(const Key('new-releases-no-more')), findsNothing);
+    expect(gridDramas(), hasLength(40));
 
     position.jumpTo(0);
     await tester.pump();
@@ -262,10 +264,7 @@ void main() {
     await tester.pumpAndSettle();
     expect(repository.fetchCalls, beforeRefresh + 1);
     expect(position.pixels, 0);
-
-    await tester.drag(list, const Offset(0, -400));
-    await tester.pump();
-    expect(_countKeys('home-drama-card-'), 20);
+    expect(gridDramas(), hasLength(20));
   });
 
   testWidgets('should_refresh_reserve_without_load_more', (tester) async {
@@ -327,14 +326,6 @@ ScrollPosition _scrollPosition(WidgetTester tester, Key listKey) {
   return tester.state<ScrollableState>(scrollable.first).position;
 }
 
-int _countKeys(String prefix) => find
-    .byWidgetPredicate((widget) {
-      final key = widget.key;
-      return key is ValueKey<String> && key.value.startsWith(prefix);
-    })
-    .evaluate()
-    .length;
-
 class _CountingCatalogRepository extends FakeCatalogRepository {
   _CountingCatalogRepository({super.dramas});