feat: SongSelect panel — NAS song list with download/play in Menu.unity

- Add DownloadManager, SongLibrary, SongSelectManager, SongDetailPanel scripts
- Rebuild SongSelect panel inside Menu.unity using VRBeatsKit style:
  left scroll list (ALL/OWNED tabs) + right detail panel (diff buttons, Download/Delete/Play)
- SceneBuilder replaced: only ③ Menu — Rebuild SongSelect Panel remains
- All UI text in English

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Assets/Script/SongLibrary.cs

@@ -0,0 +1,140 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using UnityEngine;
+
+public class SongLibrary : MonoBehaviour
+{
+    public static SongLibrary Instance { get; private set; }
+
+    private const string FileName = "song_library.json";
+    private static string SavePath => Path.Combine(Application.persistentDataPath, FileName);
+
+    private LibraryData _data = new LibraryData();
+
+    private void Awake()
+    {
+        if (Instance != null) { Destroy(gameObject); return; }
+        Instance = this;
+        DontDestroyOnLoad(gameObject);
+        Load();
+    }
+
+    // ── Public API ───────────────────────────────────────────
+
+    public void MarkDownloaded(string songId, string difficulty)
+    {
+        LibraryEntry entry = GetOrCreate(songId);
+        if (!entry.difficulties.Contains(difficulty))
+            entry.difficulties.Add(difficulty);
+        entry.lastAccessedAt = DateTime.UtcNow.ToString("o");
+        Save();
+    }
+
+    public void MarkDifficultyRemoved(string songId, string difficulty)
+    {
+        LibraryEntry entry = Find(songId);
+        if (entry == null) return;
+
+        entry.difficulties.Remove(difficulty);
+        if (entry.difficulties.Count == 0)
+            _data.entries.Remove(entry);
+        Save();
+    }
+
+    public void MarkSongRemoved(string songId)
+    {
+        _data.entries.RemoveAll(e => e.songId == songId);
+        Save();
+    }
+
+    public void TouchSong(string songId)
+    {
+        LibraryEntry entry = Find(songId);
+        if (entry == null) return;
+        entry.lastAccessedAt = DateTime.UtcNow.ToString("o");
+        Save();
+    }
+
+    public bool IsSongDownloaded(string songId)
+        => Find(songId) != null;
+
+    public bool IsDifficultyDownloaded(string songId, string difficulty)
+        => Find(songId)?.difficulties.Contains(difficulty) ?? false;
+
+    public List<LibraryEntry> GetAll()
+        => _data.entries;
+
+    public void ValidateWithFileSystem(DownloadManager dm, List<SongInfo> songs)
+    {
+        bool dirty = false;
+        foreach (SongInfo song in songs)
+        {
+            LibraryEntry entry = Find(song.id);
+            if (entry == null) continue;
+
+            if (!dm.IsSongDownloaded(song.id))
+            {
+                _data.entries.Remove(entry);
+                dirty = true;
+                continue;
+            }
+
+            entry.difficulties.RemoveAll(d => !dm.IsDifficultyDownloaded(song, d));
+            if (entry.difficulties.Count == 0)
+            {
+                _data.entries.Remove(entry);
+                dirty = true;
+            }
+        }
+        if (dirty) Save();
+    }
+
+    // ── 내부 구현 ─────────────────────────────────────────────
+
+    private LibraryEntry Find(string songId)
+        => _data.entries.Find(e => e.songId == songId);
+
+    private LibraryEntry GetOrCreate(string songId)
+    {
+        LibraryEntry entry = Find(songId);
+        if (entry != null) return entry;
+        entry = new LibraryEntry { songId = songId };
+        _data.entries.Add(entry);
+        return entry;
+    }
+
+    private void Load()
+    {
+        if (!File.Exists(SavePath)) return;
+        try
+        {
+            string json = File.ReadAllText(SavePath);
+            _data = JsonUtility.FromJson<LibraryData>(json) ?? new LibraryData();
+        }
+        catch (Exception e)
+        {
+            Debug.LogWarning($"[SongLibrary] 로드 실패, 초기화: {e.Message}");
+            _data = new LibraryData();
+        }
+    }
+
+    private void Save()
+    {
+        File.WriteAllText(SavePath, JsonUtility.ToJson(_data, true));
+    }
+}
+
+[Serializable]
+public class LibraryData
+{
+    public List<LibraryEntry> entries = new List<LibraryEntry>();
+}
+
+[Serializable]
+public class LibraryEntry
+{
+    public string       songId;
+    public List<string> difficulties   = new List<string>();
+    public string       lastAccessedAt;
+}