Add gaze: eye tracking as pointer input, with a probe to calibrate it

ft-gaze reads the Frame's eye tracker (SteamVR's eyetracking action and the
two gaze sets in eye-server.mmap, read only) and prints where the gaze lands
on the Frametop screens. ft-gazeprobe is a GTK playground on top of it: a
Vision Pro style calibration run, an accuracy test, refining the correction
model from logged points, and click, drag, and snap practice that learn the
tracker's error on the fly. It also follows SteamVR's eyetracking log for
restarts and the clicks SteamVR calibrates itself from, which it keeps only
in memory.

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

gaze/probe/ft-gazeprobe.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=Frametop Gaze Probe
+GenericName=Eye tracking playground
+Comment=How accurate the headset's eye tracking is on your screens, and gaze clicking with calibration
+Exec=@REPO@/gaze/probe/ft-gazeprobe
+Icon=view-visible
+Categories=Utility;Development;
+Keywords=eye;gaze;tracking;calibration;pointer;steamvr;frametop;