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>

session/frametop.conf.example

@@ -38,7 +38,7 @@ POINTER_GAZE_SHOW=1        # gaze mode, with POINTER_GAZE_DOT=moving: the dot sh
 POINTER_GAZE_MOUSE=precision # gaze mode: the left button holds back its press, the mouse steers, the release clicks (precision) | clicks right away (direct)
 POINTER_HEAD_DEADZONE=0.5  # keyboard clicks at the gaze (Meta+J, Meta+K): degrees the head turns before the held dot moves with it
 POINTER_KEY_TAP=0.25       # keyboard clicks: let go within this (s) and it clicks where the dot was at the press, and tells the gaze tracker it was right
-GAZE_TRACKER=steam         # gaze service: steam = SteamVR's eye tracker | own = our own (gaze/tracker: its frame grabber needs gaze/tracker/install.sh; calibrated with Calibrate on Frametop Input Settings' Gaze page)
+GAZE_TRACKER=auto          # gaze service: auto = our own eye tracker when it's installed (gaze/tracker/install.sh, which install.sh offers), else SteamVR's | own = ours | steam = SteamVR's. Each keeps its own calibration (Calibrate on Frametop Input Settings' Gaze page)
 GAZE_EYE=auto              # gaze service: eye bias. auto = each eye weighted by how far off it was at your recent nudges | left | right = that eye counts twice
 HANDS_SWAP_SIDES=0         # hand tracking (hands/run.sh install): 1 = the side cameras' names are swapped, which some SteamVR restarts cause (hands/tools/check_sides.py --ring tells)
 HANDS_CPUS=5,6,7           # hand tracking: the CPUs its model threads run on