fix: give the live session user access to the console VT

Xorg failed to start with:

    (EE) xf86OpenConsole: Cannot open virtual console 1 (Permission denied)

udev leaves console VTs as 620 root:tty — group tty has write-only, so
even a tty-group member can't open /dev/tty1 read+write (display managers
normally get this via logind seat ACLs, which a manually-started X
doesn't have). The session script now chowns the dedicated VT to
liveuser before starting X; the unit already Conflicts with getty@tty1 so
nothing else needs it.

installer/README.md

@@ -48,10 +48,15 @@ the live session with the installer in it.
 
 `overlays/live/etc/systemd/system/arcline-installer.service` starts
 `arcline-installer-session` on boot. Modern Xorg (trixie 21.1.x) refuses to
-run as root, so the session script starts Xorg as the dedicated unprivileged
-`liveuser` account on vt1, grants root display access (`xhost
-+SI:localuser:root`), then runs the installer as root (it needs root for
-`deploy-disk.sh`). When the installer exits, X and the session end.
+run as root, so the session script:
+
+1. hands the dedicated console VT to `liveuser` (`chown`, since udev leaves
+   VTs 620 root:tty which isn't readable by the tty group),
+2. starts Xorg as the unprivileged `liveuser` on vt1,
+3. grants root display access (`xhost +SI:localuser:root`),
+4. runs the installer as root (it needs root for `deploy-disk.sh`).
+
+When the installer exits, X and the session end.
 
 ## Manual run (for development / on a box without the ISO)