feat: DelaySum beamform plus hop process latency

Keep C920 stereo through SpeechBrain GCC-PHAT Delay-and-Sum, then
MetricGAN, and publish mono to LiveKit. Record last_dt_ms and log
process_ms/capture_ms every 50 hops for the latency plan.

tests/test_config.py

@@ -1,7 +1,11 @@
-from config import _parse_cameras
+from config import Config, _parse_cameras
 
 
 def test_parse_cameras_list():
     assert _parse_cameras("1,5") == ["1", "5"]
     assert _parse_cameras("all") == ["all"]
     assert _parse_cameras("1-3") == ["1", "2", "3"]
+
+
+def test_default_beamform_mode_auto():
+    assert Config().beamform_mode == "auto"