Gaze: install our eye tracker, prefer it, and say why a calibration dot wasn't taken

A user on a fresh install got "Calibration failed: only 0 of 21 dots" with
no reason. The installer never installed our tracker, so gaze used
SteamVR's, and the only way SteamVR's tracker rejects a dot is losing an
eye for most of the look. The panel just showed a red ring.

- install.sh: step 9/10 installs our tracker (gaze/tracker/install.sh)
  after gaze mode, yes by default; it needs sudo, so --yes runs it only
  when sudo won't prompt. If it fails, gaze keeps SteamVR's tracker.
  Configs that still say GAZE_TRACKER=steam (the old template) are asked
  whether to switch.
- GAZE_TRACKER=auto, the new default: ours when it's installed (the
  frame grabber, its unit, and ft-eyes' Python), else SteamVR's.
  ft-gazed rechecks every second, so installing it switches over. Input
  Settings lists Own tracker first as recommended, and says how to
  install it when it's missing (checking the host's /etc through
  /run/host from the dev container).
- The calibration panel has a note line, orange over the instructions:
  why a dot wasn't taken (an eye lost, a blink, the eyes disagreeing for
  SteamVR's tracker, from steady_samples' new drop counts; ft-eyes' reply
  for ours), what a click is still waiting for after 1.5 s, and a failed
  calibration's most common reason, which the Gaze page shows too.
  steady_samples keeps the same samples as before (checked on 2037
  windows of recordings); a lost eye is named before a blink, since its
  openness reads 0.

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

gaze/tracker/install.sh

@@ -2,8 +2,9 @@
 # Install (or remove) the frame grabber our own eye tracker needs: ft-eyegrab, as the system
 # service frametop-eyegrab.service. It copies the eye-camera frames, read-only, out of
 # SteamVR's eyetracking process into /dev/shm/frametop-eyes-cams for ft-eyes, and only while
-# ft-eyes wants them. The gaze service (gaze/ft-gazed) runs ft-eyes itself, when Eye tracker
-# is Own tracker or the gaze probe uses it.
+# ft-eyes wants them. The gaze service (gaze/ft-gazed) runs ft-eyes itself, when ours is the
+# tracker in use (GAZE_TRACKER=auto, the default, picks it once this is installed) or the gaze
+# probe uses it. install.sh offers this after gaze mode.
 # Needs host sudo, for the binary (/etc/frametop/ft-eyegrab, root's) and the unit: it asks for
 # the password in the terminal, on the Frame or from a PC, or runs SUDO_ASKPASS when that's set
 # (frame_sudo in scripts/_env.sh, which also takes it from the repo's .env).