Debian ISO: WROLPi banner in the graphical installer (#841)

The installer opened with Debian's "debian 13" banner.  Debian's
derivative guidelines say derivatives must not be presented as Debian,
and its trademark policy forbids implying endorsement, so the installer
now shows a WROLPi banner in the boot splash's style (pie logo, WROLPi,
"Off-grid library", purple accent).

The banner replaces rootskel-gtk's logo_debian.png and logo_debian_dark.png
(logo_installer*.png link to them) via config/includes.installer, which
live-build adds to the installer initrd.  It is 800x75 like Debian's; the
installer stretches its edge columns to the screen width, so the edges are
solid color.  check-iso.sh fails the build if the graphical installer's
initrd does not carry it.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

debian-live-config/check-iso.sh

@@ -100,6 +100,19 @@ for initrd in install/initrd.gz install/gtk/initrd.gz; do
   fi
 done
 
+# The graphical installer should show the WROLPi banner, not Debian's (see
+# config/includes.installer/usr/share/graphics/).
+BANNER_DIR="$(dirname "$0")/config/includes.installer/usr/share/graphics"
+mkdir -p "${TMP}/gtk"
+bsdtar -xOf "${ISO}" install/gtk/initrd.gz 2>/dev/null \
+  | bsdtar -xf - -C "${TMP}/gtk" usr/share/graphics 2>/dev/null
+for banner in logo_debian.png logo_debian_dark.png; do
+  if ! cmp -s "${BANNER_DIR}/${banner}" "${TMP}/gtk/usr/share/graphics/${banner}"; then
+    echo "ERROR: install/gtk/initrd.gz does not have the WROLPi installer banner ${banner}." >&2
+    FAILED=1
+  fi
+done
+
 if [ "${FAILED}" -ne 0 ]; then
   exit 1
 fi