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.

.env.example

@@ -30,6 +30,10 @@ ENHANCE_MODEL=speechbrain/metricgan-plus-voicebank
 # model window vs publish hop: 1 s context, 200 ms emit
 ENHANCE_CHUNK_S=1.0
 ENHANCE_HOP_S=0.2
+# C920 stereo Delay-and-Sum (SpeechBrain GCC-PHAT) before MetricGAN
+# auto (default): beamform if speechbrain loads, else average channels
+# force: require DelaySum_Beamformer, off: average L/R
+BEAMFORM_MODE=auto
 
 # Misc
 PUBLISH_TIMEOUT_S=60