Gaze: the first calibration no longer runs twice

Turning gaze mode on wakes our tracker, so its eyes come back just as the
first calibration opens. DON_DELAY later, "the headset went on" re-armed the
automatic calibration while it was still open, and when it ended, ft-eyes'
status was still a moment old (not calibrated), so a second one opened at
once. Seen live on 2026-10-05: 26 of 27 dots, then 27 more.

The headset going on re-arms it only when no check is open.
first-calibration-test.py now has the headset go on mid-calibration and
checks that only one opens.

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

gaze/gazecheck.py

@@ -894,7 +894,11 @@ class Checks:
             self.back_since = None  # gone again before DON_DELAY
         elif self.back_since is not None and now - self.back_since >= DON_DELAY:
             self.away, self.back_since = False, None
-            self.full_armed = True  # a calibration that closed unfinished opens again
+            if not self.check:
+                # A calibration that closed unfinished opens again. Not one still open: gaze mode
+                # coming on wakes our tracker, so its eyes come back just as the calibration
+                # opens, and re-arming then opened a second one when the first ended.
+                self.full_armed = True
             self.auto_quick("the headset went on")
         if svc.kind == "own" and now - svc.own_at < 5:
             reseat = any(e.get("reseat") for e in (svc.own.get("eyes") or {}).values())