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/Scenes/SampleScene.unity

@@ -2579,6 +2579,7 @@ GameObject:
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+  - component: {fileID: 2093584672}
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@@ -2613,6 +2614,18 @@ Transform:
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   m_Father: {fileID: 0}
   m_LocalEulerAnglesHint: {x: 0, y: 0, z: 0}
+--- !u!114 &2093584672
+MonoBehaviour:
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+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_GameObject: {fileID: 2093584669}
+  m_Enabled: 0
+  m_EditorHideFlags: 0
+  m_Script: {fileID: 11500000, guid: a3d8ad76f12545caa287f26889c674e3, type: 3}
+  m_Name: 
+  m_EditorClassIdentifier: 
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