feat: SongCreator 씬 완성 — Beat Sage URL 지원, info.dat 메타데이터 자동 추출

- BeatSageUploader: audio_url 지원(UploadFromUrl), PollAndDownload 공통화, ZIP 500 오류 3회 재시도
- BeatSageConverter: info.dat 파싱(SongMetadata), BPM 자동 감지 → 노트 타이밍 변환에 적용
- SongCreatorManager: title/BPM 필수 입력 제거, 난이도 4개 자동 선택, GenerateFlowFromUrl 버그 수정
- NasPublisher: audioPath null 허용(URL 흐름에서 로컬 파일 없는 경우 스킵)
- .gitignore/.gitattributes 초기 설정

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

Assets/Script/XRSimulatorLoader.cs

@@ -0,0 +1,31 @@
+using UnityEngine;
+
+// Automatically spawns the XR Interaction Simulator when running in the Editor or on PC.
+// Add this to any persistent GameObject in the Menu scene (e.g. VR_Manager).
+//
+// Setup:
+//   1. Attach this script to VR_Manager (or any root object) in Menu.unity
+//   2. In Package Manager → XR Interaction Toolkit → Samples → import "XR Interaction Simulator"
+//   3. Drag the imported prefab into the SimulatorPrefab field:
+//      Assets/Samples/XR Interaction Toolkit/<version>/XR Interaction Simulator/XR Interaction Simulator.prefab
+//
+// Controls (XR Interaction Simulator):
+//   Right-click drag  — rotate head
+//   G + mouse move    — move right controller
+//   Shift+G           — left controller
+//   Space             — trigger (UI click)
+public class XRSimulatorLoader : MonoBehaviour
+{
+    [SerializeField] private GameObject simulatorPrefab;
+
+    private void Awake()
+    {
+#if !UNITY_ANDROID || UNITY_EDITOR
+        if (simulatorPrefab != null)
+            Instantiate(simulatorPrefab);
+        else
+            Debug.LogWarning("[XRSimulatorLoader] simulatorPrefab is not assigned.\n" +
+                "Import the XR Interaction Simulator sample via Package Manager and assign the prefab.");
+#endif
+    }
+}