tools: let the submenu sweep's self-test be checked without starting a run

Verifying it previously meant launching the script, which then waits ~150 s for a
main menu and opens x11grab captures -- so the question "did the self-test pass?"
could not be answered without disturbing whatever else was using the display. That
is how a check-the-tool step turned into competing load during a measurement once
already.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wuu56cE8vJGTBtn1ppsk8v

tools/re-capture/submenu_focus_sweep.py

@@ -144,6 +144,12 @@ def _self_test():
 
 print("── decision-rule self-test ──", flush=True)
 _self_test()
+# Verifiable WITHOUT starting a run. Without this the only way to check the
+# self-test was to launch the script, which then proceeds to wait ~150 s for a
+# main menu and opens x11grab captures -- so "did my self-test pass?" could not be
+# answered without disturbing whatever else was using the display.
+if "--selftest-only" in sys.argv:
+    sys.exit(0)
 
 MAIN = wait_until(lambda a: main_menu_item(ring_row(img(a))) is not None
                   and 250 <= glyph(a) <= 420, "the main menu", 150)