Add a script to mount a root erofs to soft reboot into

Allows a developer to build an image with their local changes, then test it on their own system by soft rebooting into the generated *-root.erofs. This is ephemeral, and doesn't last a reboot.

set-nextroot.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2026 Thomas Duckworth <tduck@filotimoproject.org>
+# SPDX-FileCopyrightText: 2026 Nikolay Kochulin <basiqueevangelist@yandex.ru>
+
+set -euo pipefail
+
+# Mount a new root filesystem which can be used upon a soft reboot.
+# This can be used to test locally-built images without needing to setup a new VM.
+
+# To use, build an image. Then, with the produced kde-linux_*_root-*.erofs file,
+# run this script with `sudo ./set-nextroot.sh kde-linux_*_root-*.erofs`.
+
+if [[ $EUID -ne 0 ]]; then
+    echo "This script must be run as root" >&2
+    exit 1
+fi
+
+if ! grep -q "^ID=kde-linux$" /etc/os-release; then
+    echo "This script must be run on a KDE Linux system." >&2
+    exit 1
+fi
+
+if [[ $# -ne 1 ]]; then
+    echo "Usage: $0 /path/to/new/root.erofs" >&2
+    exit 1
+fi
+
+NEW_ROOT="$1"
+
+if [[ ! -f "$NEW_ROOT" ]]; then
+    echo "File $NEW_ROOT does not exist." >&2
+    exit 1
+fi
+
+mkdir /run/nextroot
+mount /dev/disk/by-designator/root /run/nextroot -o subvol=/@system
+mount /dev/disk/by-designator/root /run/nextroot/system -o subvol=/
+mount "$NEW_ROOT" /run/nextroot/usr -o X-mount.subdir=usr
+
+echo "New root filesystem mounted at /run/nextroot."
+echo "To switch to the new root, run: systemctl soft-reboot"