fix(judge): 修正分级升级与降级边界逻辑

- 升级时确保等级至少为8级
- 降级时确保等级不低于1级
- 修正多处比较操作，避免越界导致异常
- 优化提示信息中的等级显示，保证一致性
- 增强系统分级判定的鲁棒性

enlish-service/src/main/java/com/yinlihupo/enlish/service/service/judge/ExamWordsJudgeServiceImpl.java

@@ -254,9 +254,9 @@ public class ExamWordsJudgeServiceImpl implements ExamWordsJudgeService {
         boolean canUpgrade = zoneB.getAccuracy() >= 0.8 && (zoneA.getTotalCount() == 0 || zoneA.getAccuracy() >= 0.6);
         if (canUpgrade) {
             return new DiagnosisResult(
-                    anchorGrade + 1,
+                    Math.max(anchorGrade + 1, ExamWordsConstant.GRADE_8),
                     ActionType.UPGRADE,
-                    "恭喜！您对" + ExamWordsConstant.getGradeName(anchorGrade) + " 的掌握非常扎实，且具备挑战" + ExamWordsConstant.getGradeName(anchorGrade + 1) + " 的潜力。系统将为您解锁更高阶词库。"
+                    "恭喜！您对" + ExamWordsConstant.getGradeName(anchorGrade) + " 的掌握非常扎实，且具备挑战" + ExamWordsConstant.getGradeName(Math.max(anchorGrade + 1, ExamWordsConstant.GRADE_8)) + " 的潜力。系统将为您解锁更高阶词库。"
             );
         }
 
@@ -277,7 +277,7 @@ public class ExamWordsJudgeServiceImpl implements ExamWordsJudgeService {
             } else if (zoneC.getAccuracy() >= 0.6) {
                 // Zone C 勉强及格，Zone B 不及格 -> 降一级
                 return new DiagnosisResult(
-                        anchorGrade - 1,
+                        Math.max(anchorGrade - 1, ExamWordsConstant.GRADE_1),
                         ActionType.DOWNGRADE,
                         "检测到" + ExamWordsConstant.getGradeName(anchorGrade) + " 学习吃力，且" + ExamWordsConstant.getGradeName(zoneC.getGradeId()) + " 存在模糊点。建议降级回溯，巩固 " + ExamWordsConstant.getGradeName(zoneC.getGradeId()) + "。"
                 );
@@ -286,14 +286,14 @@ public class ExamWordsJudgeServiceImpl implements ExamWordsJudgeService {
                 // 检查 Zone D
                 if (zoneD.getTotalCount() > 0 && zoneD.getAccuracy() >= 0.6) {
                     return new DiagnosisResult(
-                            anchorGrade - 2,
+                            Math.max(anchorGrade - 2, ExamWordsConstant.GRADE_1),
                             ActionType.DOWNGRADE,
                             "您的" + ExamWordsConstant.getGradeName(zoneC.getGradeId()) + " 和 " + ExamWordsConstant.getGradeName(anchorGrade) + " 均存在脱节。建议从 " + ExamWordsConstant.getGradeName(zoneD.getGradeId()) + " 开始系统补漏。"
                     );
                 } else {
                     // 彻底崩盘，可能需要熔断，或者定级到更低
                     return new DiagnosisResult(
-                            Math.max(1, anchorGrade - 3),
+                            Math.max(ExamWordsConstant.GRADE_1, anchorGrade - 3),
                             ActionType.DOWNGRADE,
                             "基础薄弱，建议暂停当前进度，从低年级核心高频词重新开始。"
                     );