Derive stylix font size from a host fact instead of host overrides

In a dendritic NixOS flake, putting stylix.fonts.sizes.terminal overrides in each host file scatters font config. Cleaner: hosts only declare hardware facts (e.g. a custom option my.display.textScale defaulting to 1.0), and the shared stylix module computes the size: terminal = lib.mkDefault (builtins.floor (16 * config.my.display.textScale)). builtins.floor is needed because stylix sizes are ints and the multiplication yields a float. If the stylix module is also imported into home-manager, make sure the fact option is declared there too (e.g. via home-manager.sharedModules passing config.my), or the portable homeConfiguration fails to evaluate.