feat: SyncPoint.SameAsCurrentSegment music transition

- Add SameAsCurrentSegment mode to align new container's timeSamples with the old container's playback position, accounting for FadeInOffset
- Fix BeatClock callback burst when Restart is called with a past dspTime
- Add GetFirstLeafSource() for resolving playback position across nested containers
- Manual BeatClock.Restart replaces OnContainerEntered subscription for accurate timing with SyncPoint

Assets/Scripts/OCES/Audio/HandWritten/LongAudio/MusicContainerPlayer.cs

@@ -411,5 +411,20 @@ namespace OCES.Audio
             foreach (ContainerPlayHandle child in this.ChildHandles)
                 child.CollectActiveSources(result);
         }
+
+        /// <summary>
+        /// 获取第一个正在播放的 Leaf Segment 的 AudioSource。
+        /// 递归遍历子句柄，优先取当前层级的 ActiveSources，找不到再深入子层级。
+        /// </summary>
+        internal AudioSource GetFirstLeafSource()
+        {
+            if (this.ActiveSources.Count > 0) return this.ActiveSources[0];
+            foreach (ContainerPlayHandle child in this.ChildHandles)
+            {
+                AudioSource src = child.GetFirstLeafSource();
+                if (src) return src;
+            }
+            return null;
+        }
     }
 }