fix: 修正学习流程中的证据、复习与进度问题

- 独立尝试按课程要求校验内容，不再任意输入即算独立成功
- 复习节点改为学后第 1/3/7/14 天，之后每 30 天抽查；复习轮换情境
- 掌握状态按作答记录推导：需认识、可回忆、跨情境使用及最近两次无帮助
- 课程证据按实际用到的目标记录；跟读与课程对话只算辅助练习
- 复习成功不再降低已有状态
- 学完课程后进入下一节未完成课程；全部学完后引导巩固与阶段评估
- 新增 3 题基础定位，按结果设置起始课程
- 新增按课程开放的对话场景，首页推荐最少练习的场景
- 切换课程时清空上一课的步骤进度
- AI 返回 JSON 格式并按 JSON 回放历史；拼写校验忽略大小写与连字符；移除学习目标选择

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

kouyu_english/lib/core/app_state.dart

@@ -9,6 +9,7 @@ import 'models.dart';
 import 'a0_core.dart';
 import 'generated_content.dart';
 import 'local_store.dart';
+import 'review_feedback.dart';
 import 'seed_courses.dart';
 import 'ai_config.dart';
 import 'ai_service.dart';
@@ -32,6 +33,10 @@ abstract class _AppStateData extends ChangeNotifier {
   bool keepRecordings = false;
   int completedLessons = 0;
   String activeLessonId = 'a0-01';
+
+  /// Lesson recommended by the placement check; it and earlier lessons are
+  /// open without finishing the lesson before them.
+  String placementStartLessonId = 'a0-01';
   final Set<String> completedLessonIds = {};
   final Set<String> completedSegmentIds = {};
   final Set<String> reportedAiVariantKeys = {};
@@ -212,6 +217,16 @@ class AppState extends _AppStateData
     notifyListeners();
   }
 
+  /// Applies the placement result. Only A0 starting points exist until A1
+  /// content is frozen, and placement never counts as passing a lesson.
+  void setPlacementStartLesson(String lessonId) {
+    if (!a0SeedLessons.any((lesson) => lesson.id == lessonId)) return;
+    placementStartLessonId = lessonId;
+    activeLessonId = lessonId;
+    _resetLessonFlow();
+    notifyListeners();
+  }
+
   void setAiProvider(AiProviderType value) {
     aiProvider = value;
     notifyListeners();
@@ -236,6 +251,7 @@ class AppState extends _AppStateData
   void clearProgress() {
     completedLessons = 0;
     activeLessonId = 'a0-01';
+    placementStartLessonId = 'a0-01';
     completedLessonIds.clear();
     completedSegmentIds.clear();
     reportedAiVariantKeys.clear();