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 = ...; }

flake.nix

@@ -69,7 +69,7 @@
       ...
     }:
     let
-      fleet = import ./glue/fleet.nix { inherit inputs; };
+      fleet = self.lib.mkFleet { inherit inputs; };
       linuxSystem = "x86_64-linux";
       artifacts = import ./installer/artifacts.nix {
         inherit inputs fleet self;
@@ -90,11 +90,14 @@
       nixosConfigurations = fleet.nixosConfigurations;
 
       # Expose artifacts to all systems, but they are always built for x86_64-linux
-      packages = forAllSystems (_: artifacts);
+      packages.${linuxSystem} = artifacts;
 
       # Expose host type modules and installer modules for external use
       nixosModules = import ./installer/modules.nix { inherit inputs; };
 
+      # Library functions
+      lib = import ./lib { inherit inputs; };
+
       # Templates for external configurations
       templates = import ./templates;
     };