Collects all four artifacts under <repo>/dist/ (PyInstaller and Flatpak
renamed to testium-<version>(.suff); wheel and AppImage keep PEP 427 /
appimage-builder original names). Re-uses scripts/build_env.sh and
set_env.sh, same venv as run.sh. AppImage build.sh now picks the actual
output file dynamically instead of a hardcoded lowercase name.
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.
