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>

docs/design.md

@@ -82,6 +82,8 @@ A few overlays need special handling:
 
 Head follow is experimental and off by default. It works, but it's only lightly tested, and the feel is mostly a matter of its settings; polishing it is left open. With it on (`POINTER_FOLLOW=1`, or a mouse button mapped to Head follow on/off), the cursor rides on a reference direction, where you were facing when your head last settled, and keeps its offset from it. The mouse can put the cursor anywhere up to `POINTER_FOLLOW_REACH` (70 degrees) from the reference, a corner of your view included. While your head stays within `POINTER_LEASH_DEG` of the reference, nothing moves on its own. Once your head has been past the leash for `POINTER_LEASH_DELAY` (0.2 s, so a glance out and back doesn't count), the reference eases to where you're facing (time constant `POINTER_LEASH_RETURN`, 0.2 s), never falling further behind than the leash, and the cursor ends up back where it was in your view. Then it waits for the leash again. Two earlier versions didn't work out. Moving the reference only while your head pulled at the end of the leash left it up to the leash off after you turned back, and getting it centred again meant overshooting with your head. Easing it toward your facing all the time moved the cursor on every small head movement. A leash of 0 makes the reference your facing direction, so the cursor is locked to your view, and mouse movement shifts it within the view. Head roll is ignored, so tilting your head doesn't swing the cursor around. While the left button is held the cursor stays put in the room, so your head can't nudge a click or a drag. When you let go, it carries on from where it is instead of jumping.
 
+Gaze mode is experimental and off by default (`POINTER_GAZE=1`, `gaze/ft-gazectl on`, or a mouse button mapped to Gaze pointer on/off). It's MAGIC pointing (Zhai, Morimoto and Ihde, 1999): the pointer goes where you look, and the mouse does the last bit. The gaze service (`gaze/ft-gazed`) sends the helper the corrected gaze at 90 Hz, and while the gaze has the pointer, the cursor ray is that gaze from the eye. The pointer is aimed at the gaze each frame, not steered toward it, so nothing can pile up. An earlier try in the gaze probe steered the pointer with relative moves, and lost it when the pointer went idle or a controller had the laser. Moving the mouse takes the pointer from the gaze. Looking more than `POINTER_GAZE_RETAKE` (5 degrees) away from it, with the mouse still, gives it back, so small eye movements around the pointer don't pull it off what you're doing. A mouse nudge of up to `POINTER_GAZE_NUDGE_MAX` (8 degrees) before a click is sent to the gaze service as a lesson: you were looking at where you clicked when the mouse took over, so the nudge is the eye tracker's error there. Using it is what calibrates it. See `gaze/README.md` for the service, the calibration, and what was measured.
+
 Replacing a loaded driver's files, as re-running the installer used to do, leaves SteamVR honoring the virtual controller's hand role but not its laser claim: the dashboard pointer stays unassigned until SteamVR restarts. The driver installer now leaves an unchanged driver in place.
 
 `dashboard.laserRayWidthScale` controls the beam's width, but SteamVR only applies a change from its own settings screen or at restart, so it can't be switched per device while running.