存档重构，游戏内容解锁机制；教程完善（未完成）

Assets/Scripts/Menu/MenuInformationRecorder.cs

@@ -24,29 +24,70 @@ namespace Ichni.Menu
             }
         }
 
-        public SongSelectionRecord GetRecordOfThisChapter()
+        /// <summary>
+        /// 取得当前 Chapter 的选曲缓存；缓存缺失时创建“第一首有效歌曲 + 难度位置 0”的默认记录。
+        /// <para>此方法只保证缓存对象可用，不保证该歌曲或难度仍能游玩。歌曲失效与难度回退由选曲 UI 在
+        /// 每次进入页面时完成，避免把运行期缓存变成持久化存档。</para>
+        /// </summary>
+        public bool TryGetRecordOfThisChapter(out SongSelectionRecord record)
         {
+            record = null;
             ChapterSelectionUnit currentChapter = ChapterSelectionManager.instance.currentChapter;
-            if (songSelectionRecords.TryGetValue(currentChapter, out SongSelectionRecord record))
+            if (currentChapter == null || currentChapter.songs == null)
             {
-                return record;
+                return false;
             }
-            else
+
+            if (songSelectionRecords.TryGetValue(currentChapter, out SongSelectionRecord cachedRecord) && cachedRecord != null)
             {
-                return new SongSelectionRecord(currentChapter.songs[0], currentChapter.songs[0].difficultyDataList[0]);
+                record = cachedRecord;
+                return true;
             }
+
+            SongItemData firstValidSong = currentChapter.songs.Find(song => song != null);
+            if (firstValidSong == null)
+            {
+                return false;
+            }
+
+            record = new SongSelectionRecord(firstValidSong, 0);
+            songSelectionRecords[currentChapter] = record;
+            return true;
+        }
+
+        /// <summary>
+        /// 将缓存修正为当前 Chapter 中实际存在的歌曲，并保留调用方提供的难度列表偏好位置。
+        /// 这里只写内存缓存，玩家重启游戏后不会恢复该选择。
+        /// </summary>
+        public void SetRecordForChapter(ChapterSelectionUnit chapter, SongItemData song, int difficultyListIndex)
+        {
+            if (chapter == null || song == null)
+            {
+                return;
+            }
+
+            songSelectionRecords[chapter] = new SongSelectionRecord(song, difficultyListIndex);
         }
     }
 
     public class SongSelectionRecord
     {
         public SongItemData song;
-        public int difficultyIndex;
-        
-        public SongSelectionRecord(SongItemData song, DifficultyData difficulty)
+        public int difficultyListIndex;
+
+        /// <summary>
+        /// 使用难度在歌曲列表中的位置构建缓存。
+        /// 这是选曲 UI 的临时偏好，不等同于 DifficultyData.saveDifficultyId 的游戏记录稳定 ID。
+        /// </summary>
+        public SongSelectionRecord(SongItemData song, int difficultyListIndex)
         {
             this.song = song;
-            difficultyIndex = song.difficultyDataList.IndexOf(difficulty);
+            this.difficultyListIndex = difficultyListIndex;
+        }
+        
+        public SongSelectionRecord(SongItemData song, DifficultyData difficulty)
+            : this(song, song?.difficultyDataList?.IndexOf(difficulty) ?? 0)
+        {
         }
     }
-}
+}