新增：首页 Feed 接入真实数据——四态/尾部三态/聚合曝光埋点，社区 demo 消亡第一页（T3-14）

- home_page Feed segment 改接 CommunityController：骨架屏首载、空态 CTA、
  错误横幅重试、下拉刷新（失败保留旧列表 + SnackBar）、触底游标翻页
  （失败态只走显式重试，不随滚动风暴自动重打）
- PostCard 三形态（单图通栏 4:3 / 多图折叠封面 +N 角标 / 纯文字 6 行）+
  PostMediaGrid（列数规则 + ink 80% scrim 精算）+ LikeButton（error 族修订
  Colors.red，T3-14 纯展示禁用态，ToggleSync 接线留 T3-15/16）+ FeedSkeleton
  （呼吸动效尊重减弱动态设置）；降级作者统一「宠友」占位
- SignedNetworkImage：预签名 URL 缓存 key 剥离 X-Amz-* 签名参数，
  RemoteImage 全仓换用（同图不同签名命中同一缓存条目）
- feed 域埋点：feed_viewed 聚合曝光（浏览段开/结算于切 Tab、切分段、
  退后台、销毁；≥50% 可见 ≥500ms 段内按帖去重，postId 不上报）+
  feed_load_failed（refresh/load_more 双路，会话失效不报）
- integration_test/feed_live_test.dart：compose 六容器 Linux 桌面真链路
  实测入口（PATBOND_FEED_LIVE=1 门控，默认跳过）
- 测试 379 → 421（+42）：四态/尾部三态/翻页不丢不重/降级作者/曝光结算
  时机/缓存 key/事件属性全覆盖；analyze 0，format 无 diff

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/app/app.dart

@@ -14,6 +14,7 @@ import 'package:patbond_flutter/features/auth/session_manager.dart';
 import 'package:patbond_flutter/features/auth/splash_page.dart';
 import 'package:patbond_flutter/features/community/community_controller.dart';
 import 'package:patbond_flutter/features/community/community_repository.dart';
+import 'package:patbond_flutter/features/community/feed_analytics.dart';
 import 'package:patbond_flutter/features/main/main_shell_page.dart';
 import 'package:patbond_flutter/features/pets/health_record_analytics.dart';
 import 'package:patbond_flutter/features/pets/pet_analytics.dart';
@@ -48,6 +49,7 @@ class _AppState extends State<App> {
   late final CommunityController communityController;
   late final PetAnalytics petAnalytics;
   late final HealthRecordAnalytics healthRecordAnalytics;
+  late final FeedAnalytics feedAnalytics;
   late final SessionTracker _sessionTracker;
   late final AnalyticsService _analytics;
   late final PageViewTracker _pageViewTracker;
@@ -93,12 +95,13 @@ class _AppState extends State<App> {
     petsController = PetsController(
       repository: widget.petsRepository ?? _buildPetsRepository(),
     );
-    // T3-12 只装配数据层；Feed segment 的 UI 接线在 T3-14 挂入主壳。
+    // T3-14：Feed segment 接线主壳（数据层 T3-12 就位）。
     communityController = CommunityController(
       repository: widget.communityRepository ?? _buildCommunityRepository(),
     );
     petAnalytics = PetAnalytics(_analytics.trackEvent);
     healthRecordAnalytics = HealthRecordAnalytics(_analytics.trackEvent);
+    feedAnalytics = FeedAnalytics(_analytics.trackEvent);
 
     // 认证状态切换补点（根路由 AnimatedSwitcher 无路由事件）
     sessionManager.addListener(_reportAuthStateChange);
@@ -231,8 +234,10 @@ class _AppState extends State<App> {
           key: const ValueKey('shell'),
           appState: appState,
           petsController: petsController,
+          communityController: communityController,
           petAnalytics: petAnalytics,
           healthRecordAnalytics: healthRecordAnalytics,
+          feedAnalytics: feedAnalytics,
           pageViewTracker: _pageViewTracker,
           onLogout: authRepository.logout,
         );