fix: harden school imports and add optional feedback diagnostics

mobile/lib/features/school/undergraduate/session.dart

@@ -6,6 +6,7 @@ import 'package:webview_flutter/webview_flutter.dart';
 import '../../../data/json_schedule_import.dart';
 import '../../../domain/exam_schedule.dart';
 import '../../../domain/undergraduate_exam_query.dart';
+import '../../../services/diagnostic_log.dart';
 import 'navigation_policy.dart';
 import 'page_probe.dart';
 import 'timetable_adapter.dart';
@@ -98,7 +99,9 @@ class UndergraduateSession extends ChangeNotifier {
                 uri,
                 isMainFrame: request.isMainFrame,
               )) {
-            if (request.isMainFrame) _failure('学校跳转未完成，请重新认证。');
+            if (request.isMainFrame) {
+              _failure('学校跳转未完成，请重新认证。', code: 'blocked_navigation');
+            }
             return NavigationDecision.prevent;
           }
           if (request.isMainFrame &&
@@ -107,7 +110,13 @@ class UndergraduateSession extends ChangeNotifier {
           }
           return NavigationDecision.navigate;
         },
-        onPageStarted: (_) {
+        onPageStarted: (url) {
+          DiagnosticLog.instance.navigation('undergraduate', 'started', url);
+          // Within a batch, type-to-type loads retain their generation. Once
+          // published, any new document invalidates the deferred save result.
+          if (pendingExams != null) {
+            _reset();
+          }
           _document++;
           _poll?.cancel();
           _probeKey = null;
@@ -125,7 +134,11 @@ class UndergraduateSession extends ChangeNotifier {
               d = _document,
               current = await web.currentUrl();
           if (_valid(g) && d == _document && error.url == current) {
-            _failure('学校页面加载未完成，请检查网络后重试。');
+            _failure(
+              '学校页面加载未完成，请检查网络后重试。',
+              code: 'web_resource_error',
+              status: error.errorCode,
+            );
           }
         },
         onHttpError: (error) async {
@@ -135,12 +148,20 @@ class UndergraduateSession extends ChangeNotifier {
           if (_valid(g) &&
               d == _document &&
               error.request?.uri.toString() == current) {
-            _failure('学校请求未完成，请稍后重试。');
+            _failure(
+              '学校请求未完成，请稍后重试。',
+              code: 'http_error',
+              status: error.response?.statusCode,
+            );
           }
         },
       ),
     );
   Future<void> openTimetable() async {
+    DiagnosticLog.instance.add(
+      'login_opened',
+      fields: {'school': 'undergraduate', 'phase': 'timetable'},
+    );
     await _logoutFuture;
     if (_disposed) return;
     _reset();
@@ -153,6 +174,14 @@ class UndergraduateSession extends ChangeNotifier {
   }
 
   Future<void> openExams(UndergraduateExamQuery value) async {
+    DiagnosticLog.instance.add(
+      'login_opened',
+      fields: {
+        'school': 'undergraduate',
+        'phase': 'exams',
+        'term': value.term.id,
+      },
+    );
     await _logoutFuture;
     if (_disposed) return;
     _reset();
@@ -196,7 +225,16 @@ class UndergraduateSession extends ChangeNotifier {
     }
   }
 
-  void _failure(String text) {
+  void _failure(String text, {String code = 'read_failure', int? status}) {
+    DiagnosticLog.instance.add(
+      'school_read_failed',
+      fields: {
+        'school': 'undergraduate',
+        'code': code,
+        'status': status,
+        'term': query?.term.id,
+      },
+    );
     if (_disposed) return;
     _reset();
     _failed = true;
@@ -233,6 +271,7 @@ class UndergraduateSession extends ChangeNotifier {
         uri,
       );
   Future<void> _loaded(String url) async {
+    DiagnosticLog.instance.navigation('undergraduate', 'finished', url);
     final g = _generation, d = _document, key = '$_generation:$_document';
     if (!_valid(g) || _failed || _complete || _probeKey == key) return;
     _probeKey = key;
@@ -245,6 +284,14 @@ class UndergraduateSession extends ChangeNotifier {
         return;
       }
       final status = await _evidence(uri, false);
+      DiagnosticLog.instance.add(
+        'school_evidence',
+        fields: {
+          'school': 'undergraduate',
+          'result': status.authenticated,
+          'count': status.pages.length,
+        },
+      );
       if (!_valid(g) || d != _document) return;
       _loading = false;
       if (!status.authenticated) {
@@ -283,6 +330,14 @@ class UndergraduateSession extends ChangeNotifier {
       _itemTimeout = null;
       if (_index == query!.orderedTypes.length) {
         pendingExams = UndergraduateExamAdapter.merge(query!, _parts);
+        DiagnosticLog.instance.add(
+          'school_parsed',
+          fields: {
+            'school': 'undergraduate',
+            'phase': 'exams',
+            'count': pendingExams!.records.length,
+          },
+        );
         _complete = true;
         _cancelTimers();
         message = '考试已全部读取，正在保存…';
@@ -291,7 +346,9 @@ class UndergraduateSession extends ChangeNotifier {
         await _requestExam(g);
       }
     } catch (error) {
-      if (_valid(g) && d == _document) _failure('$error');
+      if (_valid(g) && d == _document) {
+        _failure('$error', code: error.runtimeType.toString());
+      }
     } finally {
       if (_probeKey == key) _probeKey = null;
     }
@@ -300,16 +357,24 @@ class UndergraduateSession extends ChangeNotifier {
   Future<void> _requestExam(int g) async {
     if (!_valid(g) || query == null || _complete) return;
     final type = query!.orderedTypes[_index];
+    DiagnosticLog.instance.add(
+      'school_read_requested',
+      fields: {
+        'school': 'undergraduate',
+        'term': query!.term.id,
+        'type': type.id,
+      },
+    );
     _expected = query!.uriFor(type);
     _loading = true;
     message = '正在读取${type.label}（${_index + 1}/${query!.types.length}）';
     _emit();
     _itemTimeout?.cancel();
     _itemTimeout = Timer(const Duration(seconds: 45), () {
-      if (_valid(g)) _failure('${type.label}读取超时，请重试。');
+      if (_valid(g)) _failure('${type.label}读取超时，请重试。', code: 'item_timeout');
     });
     _batchTimeout ??= Timer(const Duration(seconds: 120), () {
-      if (_valid(g)) _failure('考试读取超时，请重试。');
+      if (_valid(g)) _failure('考试读取超时，请重试。', code: 'batch_timeout');
     });
     await _navigate(_expected!);
   }