Make the eyes follow detected faces; start the face at power-up

hermes_face now subscribes to /vision/faces and glances toward the best face (a
recognised one first, then the biggest), smoothed, holding 2.5 s and drifting
back to centre when none is seen. Sleeping ignores gaze. Image x flips on the
display: the camera looks the way the robot faces, so a person on the image's
right is on the robot's right, the viewer's left.

Fix the pan sign carried over from the prototype: positive pan is the robot's
left (the MCP contract), which is the viewer's right on the display.

robot-face.service starts the face at boot and restarts it on failure, with a
private working directory because lgpio drops a notification pipe in its cwd and
systemd's default / is not writable. robot-cpu-performance.service holds the CPU
at full clock, which halves the face's CPU cost.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AFN8nRRCiDiMN7ChBpHJhs

pi/install-face-service.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+# Make the face start whenever the robot is powered. Run as root from this directory. Re-runnable.
+set -euo pipefail
+cd "$(dirname "$0")"
+install -m 644 robot-cpu-performance.service /etc/systemd/system/
+install -m 644 robot-face.service /etc/systemd/system/
+systemctl daemon-reload
+systemctl enable --now robot-cpu-performance.service
+systemctl enable robot-face.service
+systemctl restart robot-face.service
+sleep 8
+systemctl --no-pager --lines=4 status robot-face.service

pi/robot-cpu-performance.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Hold the CPU at full clock (the face costs about half the CPU at 1.8 GHz as under ondemand)
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/bin/sh -c 'for g in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do echo performance > "$g"; done'
+
+[Install]
+WantedBy=multi-user.target

pi/robot-face.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Robot face (hermes_face): the display is always on while the robot is powered
+After=local-fs.target robot-cpu-performance.service
+
+[Service]
+User=hermes
+# lgpio (the GPIO backend for the display) drops a notification pipe, .lgd-nfy*, in its working
+# directory, which must be writable: systemd's default, /, is not.
+RuntimeDirectory=robot-face
+WorkingDirectory=/run/robot-face
+ExecStart=/bin/bash -c 'source /home/hermes/robot-env.sh && exec ros2 run hermes_face face_node'
+Restart=always
+RestartSec=2
+
+[Install]
+WantedBy=multi-user.target