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/VRBeatsKit/Modules/VRSDK/Collections/Buffer.cs

@@ -0,0 +1,56 @@
+using System.Collections.Generic;
+
+namespace VRSDK.Collections
+{
+    public class Buffer<T>
+    {
+        private List<T> elements;
+        private int size;
+
+        public T this[int index]
+        {
+            get
+            {
+                return elements[index];
+            }
+
+            set
+            {
+                elements[index] = value;
+            }
+        }
+
+        public int Count { get { return elements.Count; } }
+
+        public Buffer(int size)
+        {
+            this.size = size;
+            elements = new List<T>( size );
+        }
+
+        public void Add(T item)
+        {
+            elements.Add( item );
+
+            if (elements.Count > size)
+            {
+                elements.RemoveAt( 0 );
+            }
+        }
+
+        public void Remove(T item)
+        {
+            elements.Remove( item );
+        }
+
+        public List<T> Sample(int size)
+        {
+            if (size > elements.Count)
+                return elements;
+
+            return elements.GetRange(elements.Count - size , size);
+        }
+    }
+
+}
+

Assets/VRBeatsKit/Modules/VRSDK/Collections/Buffer.cs.meta

@@ -0,0 +1,18 @@
+fileFormatVersion: 2
+guid: 770e28f01d6a3e14b86b7d8bf383c36c
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
+AssetOrigin:
+  serializedVersion: 1
+  productId: 168243
+  packageName: VR Beats Kit
+  packageVersion: 2.0
+  assetPath: Assets/VRBeatsKit/Modules/VRSDK/Collections/Buffer.cs
+  uploadId: 546658