fix(kiosk): stop GDM covering the wall with a greeter after auto-login

Auto-login succeeded and the wall kept streaming, but about ten seconds
after the kiosk session came up GDM autospawned a greeter on the same GPU
and a login screen covered it. gnome-kiosk ships X-GDM-SessionRegisters=false,
so nothing ever registers and GDM concludes the display is still free — which
is why every remote check (loginctl, the mirror roster) stayed green while
the physical screen showed a password prompt.

Override that session file with the flag set to true, keeping the file name
and session id identical: inventing an id makes GDM quietly fall back to the
plain GNOME session, top bar and all.

Add a per-minute guard that reclaims the foreground VT, because this class
of failure is invisible from the outside, and have the checker assert both
the foreground VT and the greeter-session count. That count had a silent
false green: journalctl has no --no-legend, so the check read an empty
stream and passed unconditionally.

Also keep a copy of the payload in /usr/local/share/mirror-kiosk. /tmp is
cleared at every boot, which deletes the checker just after the one event
you are most likely to want to re-run it for.

Verified on the reference client: a cold boot now creates no greeter session
at all, the kiosk compositor and Firefox come up undisturbed, and a console
stolen with chvt is taken back within a minute.

LLM-Generated: true

docs/kiosk-silverblue/mirror-kiosk-console-guard.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Keep the mirror kiosk on the console
+# Nothing else may be mid-switch when we look at the console.
+After=graphical.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/libexec/mirror-kiosk-console-guard