Preserve courses with pending rooms and report precise graduate import errors

mobile/lib/features/school/school_session.dart

@@ -174,6 +174,8 @@ class SchoolSession extends ChangeNotifier {
   bool _routeClosed = false;
   int _pollsLeft = 0;
   int? _probeGeneration;
+  UnsupportedSchoolLayout? _lastLayoutFailure;
+  int? _layoutFailureGeneration;
   SchoolConnectionState connectionState = SchoolConnectionState.signedOut;
   bool _loading = false,
       hasOpened = false,
@@ -334,9 +336,16 @@ class SchoolSession extends ChangeNotifier {
   String get _findingMessage => requestingExams ? '正在查找考试安排，请稍候' : '正在查找课表，请稍候';
   void _armTimeout() {
     _timeout?.cancel();
-    _timeout = Timer(
-      const Duration(seconds: 45),
-      () => _failure('查找超时，请检查学校页面后重试。已有数据保持不变。', code: 'read_timeout'),
+    _timeout = Timer(const Duration(seconds: 45), _finishReadFailure);
+  }
+
+  void _finishReadFailure() {
+    final error = _layoutFailureGeneration == _pageGeneration
+        ? _lastLayoutFailure
+        : null;
+    _failure(
+      error == null ? '查找超时，请检查学校页面后重试。已有数据保持不变。' : '课表读取未完成：$error',
+      code: error?.code ?? 'read_timeout',
     );
   }
 
@@ -396,7 +405,7 @@ class SchoolSession extends ChangeNotifier {
     }
     _poll?.cancel();
     if (_pollsLeft-- <= 0) {
-      if (finding) _failure('查找超时，请检查学校页面后重试。已有数据保持不变。', code: 'read_timeout');
+      if (finding) _finishReadFailure();
       return;
     }
     _poll = Timer(const Duration(milliseconds: 750), () {
@@ -500,6 +509,8 @@ class SchoolSession extends ChangeNotifier {
             // gets only its recognized fragment with the canonical table path.
             final draft = await _reader.read(_target, () async => page.html);
             if (!hasOpened || generation != _pageGeneration) return;
+            _lastLayoutFailure = null;
+            _layoutFailureGeneration = null;
             if (draft.schoolTerm != selectedTerm) {
               DiagnosticLog.instance.add(
                 'term_mismatch',
@@ -528,11 +539,24 @@ class SchoolSession extends ChangeNotifier {
           notifyListeners();
           return;
         } catch (error) {
+          if (generation != _pageGeneration) return;
+          if (error is UnsupportedSchoolLayout) {
+            _lastLayoutFailure = error;
+            _layoutFailureGeneration = generation;
+            message =
+                '课表暂未识别：${error.description}。正在等待页面加载完整；若仍失败，请在反馈中附带学校HTML和日志。';
+          }
           DiagnosticLog.instance.add(
             'school_parse_rejected',
             fields: {
               'school': 'graduate',
-              'code': error.runtimeType.toString(),
+              'code': error is UnsupportedSchoolLayout
+                  ? error.code
+                  : error.runtimeType.toString(),
+              if (error is UnsupportedSchoolLayout && error.row != null)
+                'row': error.row,
+              if (error is UnsupportedSchoolLayout && error.column != null)
+                'column': error.column,
             },
           );
           if (generation != _pageGeneration) return;
@@ -595,6 +619,7 @@ class SchoolSession extends ChangeNotifier {
         liveUrl != uri.toString()) {
       throw UnsupportedSchoolLayout();
     }
+    UnsupportedSchoolLayout? rejected;
     for (final page in evidence.pages) {
       if (page.kind != 'timetable') continue;
       SchoolPageDiagnostic.instance.remember(page.html, page.url);
@@ -608,11 +633,12 @@ class SchoolSession extends ChangeNotifier {
         }
         if (draft.schoolTerm != selectedTerm) continue;
         return draft;
-      } on UnsupportedSchoolLayout {
+      } on UnsupportedSchoolLayout catch (error) {
         if (generation != _pageGeneration) rethrow;
+        rejected = error;
       }
     }
-    throw UnsupportedSchoolLayout();
+    throw rejected ?? const UnsupportedSchoolLayout(code: 'table_missing');
   }
 
   Future<void> readCurrentPage() async {