refactor: Move macCaseBuilder into athenix.lib

lib/default.nix

@@ -1,4 +1,8 @@
-{ ... }:
+{
+  lib,
+  ...
+}:
 {
   mkFleet = import ./mkFleet.nix;
+  macCaseBuilder = import ./macCaseBuilder.nix { inherit lib; };
 }

lib/macCaseBuilder.nix

@@ -0,0 +1,33 @@
+{ lib }:
+let
+  # Default MAC address to station number mapping
+  defaultHostmap = {
+    "00:e0:4c:46:0b:32" = "1";
+    "00:e0:4c:46:07:26" = "2";
+    "00:e0:4c:46:05:94" = "3";
+    "00:e0:4c:46:07:11" = "4";
+    "00:e0:4c:46:08:02" = "5";
+    "00:e0:4c:46:08:5c" = "6";
+  };
+
+  # macCaseBuilder: builds a shell case statement from a hostmap
+  # Parameters:
+  #   varName: the shell variable to assign
+  #   prefix: optional string to prepend to the value (default: "")
+  #   hostmap: optional attribute set to use (default: built-in hostmap)
+  #
+  # Example:
+  #   macCaseBuilder { varName = "STATION"; prefix = "nix-"; }
+  #   # Generates case statements like: 00:e0:4c:46:0b:32) STATION=nix-1 ;;
+  builder =
+    {
+      varName,
+      prefix ? "",
+      hostmap ? defaultHostmap,
+    }:
+    lib.concatStringsSep "\n" (
+      lib.mapAttrsToList (mac: val: "      ${mac}) ${varName}=${prefix}${val} ;;") hostmap
+    );
+in
+# Export the builder function with hostmap as an accessible attribute
+lib.setFunctionArgs builder { } // { hostmap = defaultHostmap; }