feat: known-good CPU audio + 1080p VAAPI + KMS display wall

Working snapshot before the OpenVINO MetricGAN GPU spike:
thread-capped SpeechBrain DelaySum+MetricGAN on CPU, hop 200 ms,
video hold for A/V sync, Arc VAAPI H.264, headless kmssink wall.
32 tests passing. Do not treat this as a GPU audio path.

deploy/livekit-displays.service

@@ -0,0 +1,24 @@
+[Unit]
+Description=Drive 3 DRM displays with LiveKit camera output (GStreamer kmssink)
+Documentation=file:///home/fdenkena/livekit-cameras/README.md
+After=network-online.target livekit-server.service livekit-cameras.service
+Wants=network-online.target livekit-cameras.service
+
+[Service]
+Type=simple
+WorkingDirectory=/home/fdenkena/livekit-cameras
+EnvironmentFile=-/home/fdenkena/livekit-cameras/.env
+Environment=PYTHONUNBUFFERED=1
+# Exclusive KMS access — no X/Wayland on these connectors.
+ExecStartPre=/home/fdenkena/livekit-cameras/deploy/wait-for-livekit.sh
+ExecStart=/home/fdenkena/livekit-cameras/.venv/bin/python /home/fdenkena/livekit-cameras/run_displays.py
+ExecStop=/bin/kill -TERM $MAINPID
+Restart=always
+RestartSec=5
+TimeoutStartSec=60
+TimeoutStopSec=20
+KillMode=control-group
+KillSignal=SIGTERM
+
+[Install]
+WantedBy=multi-user.target