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.

README.md

@@ -5,7 +5,8 @@ LiveKit room.
 
 Each camera becomes one LiveKit participant (`cam-01` .. `cam-20`) that
 publishes:
-- a camera video track (1280x720 @ 30 fps by default, MJPG from V4L2)
+- a camera video track (640x360 @ 30 fps MJPG, H.264 VAAPI — sized
+  for low-latency wall decode, not max quality)
 - a microphone audio track (16 kHz mono, cleaned with speechbrain)
 
 Identity is the USB serial, pinned by udev (`/dev/camNN` + `slots.json`).
@@ -72,28 +73,31 @@ re-run `--write` unless you intend to remap slots.
 ## Headless displays (LiveKit -> GStreamer kmssink)
 
 This host has no X11/Wayland. `run_displays.py` joins the LiveKit room as
-`display-wall` (subscribe-only) and pipes each camera's video into a
-GStreamer `kmssink` on a DRM connector.
+`display-wall` (subscribe-only) and drives three DRM roles with GStreamer
+`kmssink`:
 
-Default mapping: `cam-01`, `cam-02`, `cam-03` onto the first 3 DRM heads
-(connected first, Arc `xe` HDMI preferred). Today HDMI-A-7 is the live 4K
-panel; plug two more HDMI cables into the Arc card (`HDMI-A-5`/`HDMI-A-6`)
-and they light up on the same process.
+1. **grid** — 5x4 mosaic of cam-01..cam-20
+2. **speaker** — designated speaker camera (`DISPLAY_SPEAKER_CAMERA`, default
+   `cam-01`; set to `active` to follow LiveKit active speaker)
+3. **screenshare** — stub placeholder until the production LiveKit server
+   is online (no subscribe, no speaker login yet)
+
+Default connector order: connected Arc HDMI first (today HDMI-A-7 is live;
+HDMI-A-5 / HDMI-A-6 light up when cabled).
 
 ```bash
 .venv/bin/python run_displays.py --list
-.venv/bin/python run_displays.py --test-pattern   # smpte/ball/snow, no LiveKit
-.venv/bin/python run_displays.py                  # subscribe cam-01..03
-.venv/bin/python run_displays.py --connectors HDMI-A-7,HDMI-A-6,HDMI-A-5
+.venv/bin/python run_displays.py --test-pattern
+.venv/bin/python run_displays.py
+.venv/bin/python run_displays.py --speaker-camera cam-07
 
-sudo cp deploy/livekit-displays.service /etc/systemd/system/
-sudo systemctl daemon-reload
-sudo systemctl enable --now livekit-displays
+sudo systemctl restart livekit-displays
+journalctl -u livekit-displays -f
 ```
 
-`DISPLAY_IDENTITIES` / `DISPLAY_CONNECTORS` / `DISPLAY_COUNT` in `.env`.
-kmssink takes the DRM master for those connectors (the framebuffer console
-on HDMI-A-7 will be replaced by camera video).
+`.env`: `DISPLAY_ROLES`, `DISPLAY_CONNECTORS`, `DISPLAY_SPEAKER_CAMERA`,
+`DISPLAY_GRID_*`. kmssink takes the DRM master (framebuffer console on
+HDMI-A-7 is replaced by the grid).
 
 ## Audio cleanup