Add gaze mode to the pointer: it goes where you look

MAGIC pointing (Zhai et al. 1999), off by default: with POINTER_GAZE=1,
"gaze on", or a button mapped to the new gaze_toggle action, the cursor ray
is the corrected gaze from ft-gazed while the gaze has the pointer. Moving
the mouse takes the pointer from where the gaze left it; looking more than
POINTER_GAZE_RETAKE (5 deg) away with the mouse still gives it back. The
pointer is aimed at the gaze, never steered toward it, and only fresh gaze
moves it, so a stopped service or a blink leaves it where it is. A mouse
nudge of up to POINTER_GAZE_NUDGE_MAX (8 deg) before a click is sent to
ft-gazed as a lesson in the eye tracker's error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

input-settings/ft_input_settings.py

@@ -38,7 +38,8 @@ ACTION_LABELS = {
     "left": "Left click", "right": "Right click", "middle": "Middle click", "back": "Back",
     "scroll_up": "Scroll up", "scroll_down": "Scroll down", "dashboard": "Toggle SteamVR dashboard",
     "recenter": "Recenter pointer", "pointer_toggle": "Pointer on/off",
-    "follow_toggle": "Head follow on/off (experimental)", "sens_up": "Faster pointer",
+    "follow_toggle": "Head follow on/off (experimental)", "gaze_toggle": "Gaze pointer on/off (experimental)",
+    "sens_up": "Faster pointer",
     "sens_down": "Slower pointer", "layout_reset": "Reset desktop screen layout",
     "screens_toggle": "Hide/show desktop screens", "key": "Pass through as key",
     "none": "Do nothing",