feat: add lib.mkFleet for external flake consumption

- Create lib/mkFleet.nix to expose fleet generator as library function
- Allow external flakes to use Athenix's fleet logic with custom inventory
- Export lib output in flake.nix with proper input passing
- Enable usage: nixosConfigurations = athenix.lib.mkFleet { fleet = ...; hwTypes = ...; }

lib/default.nix

@@ -0,0 +1,4 @@
+{ inputs }:
+{
+  mkFleet = import ./mkFleet.nix;
+}