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