build.sh runs appimage-builder in a Debian Bookworm container (Podman or
Docker) so it works on Arch / non-Debian hosts. Uses single src/requirements.txt;
TESTIUM_VERSION exported in runtime.env.
org.testium.Testium.yaml uses host Python/Lua only (no bundled interpreter).
build.sh exports a .flatpak bundle. README documents the install procedure.
