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

ros/hermes_face/test/test_render.py

@@ -60,7 +60,7 @@ def test_pan_moves_the_eyes():
         frame, _ = run(r, 2)
         lum = frame.astype(np.float32).sum(axis=(0, 2))
         return float((lum * np.arange(W)).sum() / lum.sum())
-    assert centroid_x(-60) > centroid_x(0) > centroid_x(60)   # +pan = robot's left = viewer's right
+    assert centroid_x(60) > centroid_x(0) > centroid_x(-60)   # +pan = robot's left = viewer's right
 
 
 def test_box_blur_conserves_energy():
@@ -110,3 +110,37 @@ def test_wild_parameters_are_clamped():
 def test_unknown_phosphor_rejected():
     with pytest.raises(ValueError):
         FaceRenderer(phosphor='P99')
+
+
+def eye_centroid(r, seconds, t0=0.0):
+    frame, t = run(r, seconds, t0)
+    lum = frame[:130].astype(np.float32).sum(axis=(0, 2))         # the eyes live in the top half
+    rows = frame[:130].astype(np.float32).sum(axis=(1, 2))
+    return float((lum * np.arange(W)).sum() / lum.sum()), float((rows * np.arange(130)).sum() / rows.sum()), t
+
+
+def test_gaze_moves_the_eyes_toward_the_person_and_drifts_back():
+    r = FaceRenderer(state='idle', seed=0, jitter=0)
+    cx0, cy0, t = eye_centroid(r, 2)
+    r.set_gaze(1.0, 0.0, t)
+    cx1, _, t = eye_centroid(r, 1, t)           # held, so still glancing
+    assert cx1 > cx0 + 5
+    r.set_gaze(0.0, -1.0, t)
+    _, cy2, t = eye_centroid(r, 1, t)
+    assert cy2 < cy0 - 3                         # up on the display
+    cx3, cy3, _ = eye_centroid(r, 8, t)          # no refresh: hold expires, eyes return
+    assert abs(cx3 - cx0) < 2 and abs(cy3 - cy0) < 2
+
+
+def test_sleeping_ignores_gaze():
+    r = FaceRenderer(state='sleeping', seed=0, jitter=0)
+    cx0, _, t = eye_centroid(r, 4)
+    r.set_gaze(1.0, 0.0, t)
+    cx1, _, _ = eye_centroid(r, 2, t)
+    assert abs(cx1 - cx0) < 1.5
+
+
+def test_gaze_is_clamped():
+    r = FaceRenderer(seed=0)
+    r.set_gaze(50, -50, 0.0)
+    assert r._gaze == (1.0, -1.0)