P5-R3: fix count-in recursion + test buffer loop size

src/backing/player.py

@@ -639,7 +639,7 @@ class BackingTrackPlayer:
 
         if ptype == ParameterType.PLAY:
             if value > 0.5:
-                return self.play()
+                return self.play(count_in=False)
             else:
                 return self.stop()
 

tests/test_backing.py

@@ -1628,7 +1628,9 @@ class TestCountInIntegration:
             assert player._count_in_remaining > 0  # noqa: SLF001
 
             # Process until count-in completes
-            for _ in range(50):
+            # 1 bar × 4 beats × (60/120)s = 2.0s count-in
+            # 2.0s / (256 frames / 48000 Hz) ≈ 375 buffers needed
+            for _ in range(400):
                 buf = player.process_buffer(256)
                 if player.is_playing:
                     break