fix(iso): copy memtest86+ to ISO root via binary hook

memtest files live in chroot /boot (inside squashfs) but GRUB needs
them on the ISO filesystem. Binary hook copies them out at build time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/config/hooks/normal/9100-memtest.hook.binary

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Copy memtest86+ binaries from chroot /boot into the ISO boot directory
+# so GRUB can chainload them directly (they must be on the ISO filesystem,
+# not inside the squashfs).
+set -e
+
+for f in memtest86+x64.bin memtest86+x64.efi memtest86+ia32.bin memtest86+ia32.efi; do
+    src="chroot/boot/${f}"
+    if [ -f "${src}" ]; then
+        cp "${src}" "binary/boot/${f}"
+        echo "memtest: copied ${f} to binary/boot/"
+    fi
+done