Introduce a new `mangohud.nix` module with sensible defaults, import it
in
`modules/home/default.nix`, and add a `mangohud.enable` flag to the host
configurations (enabled for desktop and laptop, disabled for the default
host).
Comment out the default Gamescope arguments in `modules/core/steam.nix`
and
add a reference command line that includes the MangoHud flag.
