Reduce mixer and Bluetooth buffers from 500 ms to 200 ms

SBC medium, not the larger buffer, removed the headphone relay dropouts,
so cut dmix, bluealsa-aplay and alsaloop buffers to 200 ms and the
Shairport backend buffer to 0.15 s. Verified by listening (iPhone ->
headphones, with AirPlay mixed in) with no RTP loss or underruns.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_manager.py

@@ -52,7 +52,7 @@ class ManagerTests(unittest.TestCase):
         self.assertIn('Loopback', f['/etc/asound.conf'])
         self.assertIn('rate 48000',f['/etc/asound.conf'])
         self.assertIn('--sbc-quality=medium',f['/etc/systemd/system/bluealsa.service.d/10-radxa-sink.conf'])
-        self.assertIn('-t 500000',unit)
+        self.assertIn('-t 200000',unit)
 
     def test_name_quotes_are_escaped(self):
         f=m.templates(self.config(name='Desk "Two"'))