fix: ErrorOverlay 不可见 — Stack 在 Visibility(visible:false) 时坍缩为 0×0

当 _hasMainFrameError=true 时，Visibility(visible:false) 将 WebView
替换为 SizedBox.shrink()（0×0），这是 Stack 唯一的非 positioned 子组件。
Scaffold.body 提供松约束（0→max），导致 Stack 尺寸为 0×0，
Positioned.fill 的 ErrorOverlay 也变为 0×0，用户只能看到 Scaffold 的
深色背景而看不到错误页面的内容。

修复方式：用 SizedBox.expand() 包裹 Stack，强制紧约束使其始终填满屏幕。

附带变更：
- 添加 _debugLog() 封装（kDebugMode 保护）
- 在 7 个关键状态转换点添加调试日志

packages/web_shell_core/lib/src/ui/shell_page.dart

@@ -199,6 +199,12 @@ class _WebShellPageState extends State<WebShellPage>
     return generation == _webViewGeneration;
   }
 
+  void _debugLog(String message) {
+    if (kDebugMode) {
+      debugPrint(message);
+    }
+  }
+
   PlatformWebViewWidgetCreationParams _buildAndroidWidgetParams(
     PlatformWebViewWidgetCreationParams widgetParams,
     AndroidRenderMode renderMode,
@@ -282,6 +288,11 @@ class _WebShellPageState extends State<WebShellPage>
             _hasMeasuredProgress = true;
           }
         });
+        _debugLog(
+          '🔍 [onProgress] progress=$progress, '
+          'hasStartedRemote=$_hasStartedRemoteMainFrame, '
+          'hasMeasured=$_hasMeasuredProgress',
+        );
       },
       onNavigationRequest: (request) async {
         if (!_isActiveWebViewGeneration(generation)) {
@@ -316,6 +327,13 @@ class _WebShellPageState extends State<WebShellPage>
         _hasStartedRemoteMainFrame = true;
         _cancelStartupWatchdog();
         _recordWebViewEvent('页面开始加载：$url');
+        _debugLog(
+          '🔍 [onPageStarted] url=$url, '
+          'hasStartedRemote=$_hasStartedRemoteMainFrame, '
+          'hasBootstrapped=$_hasBootstrapped, '
+          'hasMainFrameError=$_hasMainFrameError, '
+          'watchdog已取消',
+        );
         if (!mounted) {
           return;
         }
@@ -341,6 +359,12 @@ class _WebShellPageState extends State<WebShellPage>
         }
         _recordWebViewEvent('页面加载完成：$url');
         _cancelStartupWatchdog();
+        _debugLog(
+          '🔍 [onPageFinished] url=$url, '
+          'hasMainFrameError=$_hasMainFrameError, '
+          'injectBridge=${!_hasMainFrameError}, '
+          'retryCount=$_startupRetryCount',
+        );
         if (!_hasMainFrameError) {
           unawaited(_injectAppShellBridge(_controller, url));
         }
@@ -392,6 +416,11 @@ class _WebShellPageState extends State<WebShellPage>
           'url=${error.url}, '
           'description=${error.description}',
         );
+        _debugLog(
+          '🔍 [onWebResourceError] isMainFrame=${error.isForMainFrame}, '
+          'errorType=${error.errorType}, '
+          'willSetMainFrameError=${error.isForMainFrame != false}',
+        );
         if (!mounted || error.isForMainFrame == false) {
           return;
         }
@@ -476,10 +505,19 @@ class _WebShellPageState extends State<WebShellPage>
   }
 
   Future<void> _handleStartupTimeout() async {
+    _debugLog(
+      '🔍 [_handleStartupTimeout] mounted=$mounted, '
+      'isLoading=$_isLoadingPage, '
+      'hasMainFrameError=$_hasMainFrameError, '
+      'hasStartedRemote=$_hasStartedRemoteMainFrame, '
+      'retryCount=$_startupRetryCount, '
+      'renderIndex=$_renderModeIndex',
+    );
     if (!mounted ||
         !_isLoadingPage ||
         _hasMainFrameError ||
         _hasStartedRemoteMainFrame) {
+      _debugLog('🔍 [_handleStartupTimeout] 提前返回，不执行超时恢复');
       return;
     }
 
@@ -487,6 +525,11 @@ class _WebShellPageState extends State<WebShellPage>
     final maxRetryCount = _androidCompatibilityPlan.prefersAggressiveRecovery
         ? 2
         : 1;
+    _debugLog(
+      '🔍 [_handleStartupTimeout] switchedRenderMode=$switchedRenderMode, '
+      'maxRetryCount=$maxRetryCount, '
+      'currentRetryCount=$_startupRetryCount',
+    );
 
     if (switchedRenderMode || _startupRetryCount < maxRetryCount) {
       final nextRetryCount = _startupRetryCount + 1;
@@ -505,6 +548,7 @@ class _WebShellPageState extends State<WebShellPage>
       return;
     }
 
+    _debugLog('🔍 [_handleStartupTimeout] 所有重试已耗尽，显示错误页面');
     _setMainFrameError(
       title: '页面启动超时',
       message: <String>[
@@ -516,6 +560,10 @@ class _WebShellPageState extends State<WebShellPage>
   }
 
   void _setMainFrameError({required String title, required String message}) {
+    _debugLog(
+      '🔍 [_setMainFrameError] title=$title, '
+      'message=$message',
+    );
     _cancelStartupWatchdog();
     if (!mounted) {
       return;
@@ -528,6 +576,12 @@ class _WebShellPageState extends State<WebShellPage>
       _errorMessage = message;
       _progress = 0;
     });
+    _debugLog(
+      '🔍 [_setMainFrameError] 状态已更新: '
+      'isLoading=$_isLoadingPage, '
+      'hasMainFrameError=$_hasMainFrameError, '
+      '→ 应显示 ErrorOverlay',
+    );
   }
 
   Future<void> _recoverFromBrokenStartupState({bool deepReset = false}) async {
@@ -1023,6 +1077,15 @@ class _WebShellPageState extends State<WebShellPage>
     final showProgressBar =
         _isLoadingPage && (!_hasMeasuredProgress || _progress < 100);
     final showLaunchOverlay = !_hasBootstrapped && !_hasMainFrameError;
+    _debugLog(
+      '🔍 [build] showProgressBar=$showProgressBar, '
+      'showLaunchOverlay=$showLaunchOverlay, '
+      'showErrorOverlay=$_hasMainFrameError, '
+      'showWebView=${!_hasMainFrameError}, '
+      'isLoading=$_isLoadingPage, '
+      'hasBootstrapped=$_hasBootstrapped, '
+      'progress=$_progress',
+    );
 
     return PopScope<void>(
       canPop: false,
@@ -1041,39 +1104,41 @@ class _WebShellPageState extends State<WebShellPage>
               end: Alignment.bottomCenter,
             ),
           ),
-          child: Stack(
-            children: <Widget>[
-              Visibility(
-                visible: !_hasMainFrameError,
-                child: _webViewWidget,
-              ),
-              if (showProgressBar)
-                Positioned(
-                  top: 0,
-                  left: 0,
-                  right: 0,
-                  child: TopProgressBar(
-                    progress: _progress,
-                    hasMeasuredProgress: _hasMeasuredProgress,
-                  ),
+          child: SizedBox.expand(
+            child: Stack(
+              children: <Widget>[
+                Visibility(
+                  visible: !_hasMainFrameError,
+                  child: _webViewWidget,
                 ),
-              if (showLaunchOverlay)
-                Positioned.fill(
-                  child: LaunchOverlay(
-                    progress: _progress,
-                    hasMeasuredProgress: _hasMeasuredProgress,
+                if (showProgressBar)
+                  Positioned(
+                    top: 0,
+                    left: 0,
+                    right: 0,
+                    child: TopProgressBar(
+                      progress: _progress,
+                      hasMeasuredProgress: _hasMeasuredProgress,
+                    ),
                   ),
-                ),
-              if (_hasMainFrameError)
-                Positioned.fill(
-                  child: ErrorOverlay(
-                    title: _errorTitle,
-                    message: _errorMessage,
-                    currentUrl: _currentUrl,
-                    onRetry: _reloadPage,
+                if (showLaunchOverlay)
+                  Positioned.fill(
+                    child: LaunchOverlay(
+                      progress: _progress,
+                      hasMeasuredProgress: _hasMeasuredProgress,
+                    ),
                   ),
-                ),
-            ],
+                if (_hasMainFrameError)
+                  Positioned.fill(
+                    child: ErrorOverlay(
+                      title: _errorTitle,
+                      message: _errorMessage,
+                      currentUrl: _currentUrl,
+                      onRetry: _reloadPage,
+                    ),
+                  ),
+              ],
+            ),
           ),
         ),
       ),