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。

lib/pages/discover/widgets/discover_search_results.dart

@@ -10,6 +10,8 @@ import '../../../l10n/app_localizations.dart';
 import 'drama_grid.dart';
 
 /// 剧场搜索结果：加载、失败与空结果都留在同一页内。
+///
+/// 自身是 sliver，好让结果网格跟着剧场页一起懒加载。
 class DiscoverSearchResults extends StatelessWidget {
   const DiscoverSearchResults({
     super.key,
@@ -26,16 +28,24 @@ class DiscoverSearchResults extends StatelessWidget {
   final VoidCallback onRetry;
   final ValueChanged<Drama> onOpen;
 
+  static const EdgeInsets _topPadding = EdgeInsets.only(top: 12);
+
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context)!;
-    return Padding(
-      padding: const EdgeInsets.only(top: 12),
-      child: _content(l10n),
+    if (!loading && error == null && results.isNotEmpty) {
+      return SliverPadding(
+        padding: _topPadding,
+        sliver: SliverDramaGrid(l10n: l10n, dramas: results, onOpen: onOpen),
+      );
+    }
+    return SliverPadding(
+      padding: _topPadding,
+      sliver: SliverToBoxAdapter(child: _state(l10n)),
     );
   }
 
-  Widget _content(AppLocalizations l10n) {
+  Widget _state(AppLocalizations l10n) {
     if (loading) {
       return const Padding(
         padding: EdgeInsets.symmetric(vertical: 80),
@@ -61,14 +71,11 @@ class DiscoverSearchResults extends StatelessWidget {
         onAction: onRetry,
       );
     }
-    if (results.isEmpty) {
-      return EmptyState(
-        key: const Key('discover-search-empty'),
-        icon: Icons.search_off_rounded,
-        title: l10n.noSearchResults,
-        subtitle: l10n.emptyHint,
-      );
-    }
-    return DramaGrid(l10n: l10n, dramas: results, onOpen: onOpen);
+    return EmptyState(
+      key: const Key('discover-search-empty'),
+      icon: Icons.search_off_rounded,
+      title: l10n.noSearchResults,
+      subtitle: l10n.emptyHint,
+    );
   }
 }

lib/pages/discover/widgets/drama_grid.dart

@@ -10,7 +10,6 @@ import '../../../core/theme/app_dimens.dart';
 import '../../../core/widgets/remote_image.dart';
 import '../../../l10n/app_localizations.dart';
 
-const int _columns = 2;
 const double _columnGap = 8;
 const double _cardGap = 14;
 const double _gridHorizontalPadding = 12;
@@ -29,8 +28,10 @@ const Color _tagChipBackground = AppColors.raised;
 final RegExp _tagSplitter = RegExp(r'[,/、·]');
 
 /// 剧场/分类页共用的双列剧目网格。
-class DramaGrid extends StatelessWidget {
-  const DramaGrid({
+///
+/// 按行懒加载：翻到第 N 页时只构建当前可见的几张卡，长列表不会把整本目录都留在树上。
+class SliverDramaGrid extends StatelessWidget {
+  const SliverDramaGrid({
     super.key,
     required this.l10n,
     required this.dramas,
@@ -43,33 +44,41 @@ class DramaGrid extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return Padding(
-      key: const Key('home-all-dramas-section'),
+    final rowCount = (dramas.length + 1) ~/ 2;
+    return SliverPadding(
       padding: const EdgeInsets.symmetric(horizontal: _gridHorizontalPadding),
-      child: Row(
-        key: const Key('home-all-dramas-grid'),
-        crossAxisAlignment: CrossAxisAlignment.start,
-        children: [
-          Expanded(child: _column(0)),
-          const SizedBox(width: _columnGap),
-          Expanded(child: _column(1)),
-        ],
+      sliver: SliverList(
+        delegate: SliverChildBuilderDelegate((context, row) {
+          final left = dramas[row * 2];
+          final rightIndex = row * 2 + 1;
+          final right = rightIndex < dramas.length ? dramas[rightIndex] : null;
+          return Padding(
+            // 行间距沿用卡片间距，首行不需要。
+            padding: EdgeInsets.only(top: row == 0 ? 0 : _cardGap),
+            child: Row(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: [
+                Expanded(child: _card(left)),
+                const SizedBox(width: _columnGap),
+                Expanded(
+                  child: right == null
+                      ? const SizedBox.shrink()
+                      : _card(right),
+                ),
+              ],
+            ),
+          );
+        }, childCount: rowCount),
       ),
     );
   }
 
-  Widget _column(int start) {
-    final children = <Widget>[];
-    for (var index = start; index < dramas.length; index += _columns) {
-      final drama = dramas[index];
-      children.add(
-        DramaGridCard(l10n: l10n, drama: drama, onTap: () => onOpen(drama)),
-      );
-      if (index + _columns < dramas.length) {
-        children.add(const SizedBox(height: _cardGap));
-      }
-    }
-    return Column(children: children);
+  Widget _card(Drama drama) {
+    return DramaGridCard(
+      l10n: l10n,
+      drama: drama,
+      onTap: () => onOpen(drama),
+    );
   }
 }