fix(gui): fire mount auto-centering on the first idle status tick

The "apply after mounting" checkbox clicked Auto Centering as soon as the
status poll showed a new sample id. /sample/mount still holds the hw lock
at that point, /alc/center_loop takes the same lock non-blocking, and the
resulting BeamlineBusyException is a quiet error the GUI swallows. So the
automatic click never did anything; only a later manual click worked.

Arm on the sample-id change and click on the first status tick with
busy=False, which is exactly what the user did by hand. Skip arming while
queue automation runs: it mounts and centers on its own and frees the lock
between /scan/auto calls, so a GUI centering in that gap would steal the
lock from the next scan. Pause during a scan's mount phase can still arm
that one sample, but Pause ends in an unmount, so the window is a ms-wide
lock gap sampled at 2 Hz; accepted, noted in the code.

Supersedes the three-commit approach on
fix/auto-center-after-mount-wait-idle (PR 243).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/unit/gui/test_data_collection_settings.py

@@ -353,6 +353,39 @@ def test_auto_center_fires_on_mount_only_when_armed(settings_panel):
     assert len(clicks) == 3
 
 
+def test_auto_center_waits_for_hw_lock_after_mount(settings_panel):
+    clicks = []
+    settings_panel.find_tip.clicked.connect(lambda: clicks.append(1))
+    settings_panel.auto_center_after_mount.setChecked(True)
+    settings_panel._track_sample(None)
+
+    # The mount still holds the hw lock when the sample id first shows up:
+    # /alc/center_loop would be refused, so no click yet.
+    settings_panel._track_sample(1, busy=True)
+    settings_panel._track_sample(1, busy=True)
+    assert not clicks
+    # First idle tick fires it, exactly once.
+    settings_panel._track_sample(1, busy=False)
+    settings_panel._track_sample(1, busy=False)
+    assert len(clicks) == 1
+
+    # Unmount while still pending drops the request.
+    settings_panel._track_sample(None)
+    settings_panel._track_sample(2, busy=True)
+    settings_panel._track_sample(None, busy=False)
+    assert len(clicks) == 1
+
+    # Queue automation mounts and centers on its own: never fire in its gaps.
+    settings_panel.set_automation_running(True)
+    settings_panel._track_sample(3, busy=True)
+    settings_panel._track_sample(3, busy=False)
+    assert len(clicks) == 1
+    settings_panel.set_automation_running(False)
+    settings_panel._track_sample(None)
+    settings_panel._track_sample(4, busy=False)
+    assert len(clicks) == 2
+
+
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