Mix inputs at 0 dB instead of -3 dB

The common attenuation made playback too quiet. Inputs are now summed
without attenuation; docs note that loud simultaneous inputs can clip.

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

tests/test_manager.py

@@ -40,7 +40,7 @@ class ManagerTests(unittest.TestCase):
         self.assertIn('"radxa_mix"',f['/etc/shairport-sync.conf'])
         self.assertIn('--pcm=radxa_mix',f['/etc/systemd/system/bluealsa-aplay.service.d/10-radxa-mixer.conf'])
         self.assertNotIn('--single-audio', ''.join(f.values()))
-        self.assertIn('ttable { 0.0 0.707945784 1.1 0.707945784 }',f['/etc/asound.conf'])
+        self.assertIn('ttable { 0.0 1.0 1.1 1.0 }',f['/etc/asound.conf'])
         self.assertIn('rate 44100',f['/etc/asound.conf'])
 
     def test_output_device_is_not_hardcoded(self):