Wait for file playback workers before restart

fix.txt

@@ -99,7 +99,7 @@ Priority 1: transport, lifecycle, and correctness
     overwrites b.Stream. Destroy/stop the old resources before reconnecting;
     make disconnect cleanup idempotent.
 
-12. File player sessions race each other
+[x] 12. File player sessions race each other
     File: fileplayback/player.go
     readFileAudio repeatedly reads mutable Player stopChan/ctx/audioChan.
     Stop can return and a new PlayFile can replace them while the old ffmpeg