Calibration all in the headset panel; the gaze probe is a development tool

Check headset fit now runs in the headset panel too ("fitcheck"): a card per
eye (tracked or lost, the tracker's signal, how much of the last 10 s it was
seen) and the hints, from the probe's fitcheck.py, updated at most twice a
second; left click or Meta+J runs its guided check, right click or Meta+K closes
it. So Quick check, Calibrate, and Check headset fit all happen in one place.

The gaze probe moves to the Gaze page's overflow menu as "Gaze probe
(development)", and its app menu entry says it's a development tool. Texts that
sent users to it ("use Calibrate… with Own tracker") point at Calibrate.

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

gaze/fitcheck.py

@@ -1,5 +1,5 @@
-"""fitcheck: how well the eye tracker sees each eye, for fitting the headset (ft-gazeprobe's
-Headset fit mode).
+"""fitcheck: how well the eye tracker sees each eye, for fitting the headset (the headset
+panel's fit check, gazecheck.py, and ft-gazeprobe's Headset fit mode).
 
 From each ft-gaze sample it takes, per eye, whether the tracker has that eye (its variance
 for the eye's direction, "unc", under EYE_LOST; see gazecal), how open the eye is, and the