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.

README.md

@@ -71,13 +71,22 @@ re-run `--write` unless you intend to remap slots.
 
 ## Audio cleanup
 
-The mic audio is cleaned per hop (default 200 ms) by
-`speechbrain/metricgan-plus-voicebank` (16 kHz, CPU) over a 1 s rolling
-window (`ENHANCE_CHUNK_S=1.0`, `ENHANCE_HOP_S=0.2`). One worker process
-per camera loads the model once and reuses it. `enhance_batch` must be
-called with `lengths=torch.tensor([1.0])` (relative, not sample count).
-`ENHANCE_MODE=off` (or a failed model load with `auto`) falls back to a
-lightweight high-pass + soft-clip DSP path so streams still work.
+C920 mics are 16 kHz **stereo**. Each hop (default 200 ms) is kept
+stereo through a SpeechBrain preprocessor, then published as mono:
+
+1. `AudioNormalizer(mix="keep")` at 16 kHz
+2. Delay-and-Sum beamform (GCC-PHAT TDOA) — the live path from the
+   SpeechBrain [multi-microphone beamforming tutorial](https://speechbrain.readthedocs.io/en/latest/tutorials/preprocessing/multi-microphone-beamforming.html)
+   (`speechbrain.lobes.beamform_multimic.DelaySum_Beamformer`)
+3. `speechbrain/metricgan-plus-voicebank` neural enhancement on a 1 s
+   rolling window (`ENHANCE_CHUNK_S=1.0`, `ENHANCE_HOP_S=0.2`)
+
+One worker process per camera loads the modules once and reuses them.
+`enhance_batch` must be called with `lengths=torch.tensor([1.0])`
+(relative, not sample count). `ENHANCE_MODE=off` (or a failed model
+load with `auto`) skips MetricGAN; `BEAMFORM_MODE=off` averages L/R
+instead of Delay-and-Sum. DSP high-pass + soft-clip is the last-resort
+fallback so streams still work.
 
 Until the 1 s window is full, hops use the DSP fallback so LiveKit is
 not silent on startup.
@@ -116,8 +125,9 @@ systemctl restart livekit-cameras   # pick up newly plugged cameras
 
 - Video uses MJPG directly from the UVC interface (`ffmpeg -f v4l2
   -input_format mjpeg`) — cheap, and converts to BGRA for LiveKit.
-- Audio: ALSA mics on these C920s deliver 16 kHz **stereo** only; the worker
-  downmixes to mono for LiveKit.
+- Audio: ALSA mics on these C920s deliver 16 kHz **stereo** only; the
+  worker beamforms (Delay-and-Sum) then enhances, and publishes **mono**
+  to LiveKit.
 - Camera identity is USB serial via `slots.json` / udev; USB port is
   fallback only for cameras not yet in the map.
 - Workers auto-restart (up to 10 times) if a camera drops.