feat(registry): add generic fleet namespace

lib/registry.nix

@@ -0,0 +1,53 @@
+{ lib }:
+let
+  inherit (lib)
+    filterAttrs
+    hasSuffix
+    mapAttrs
+    mapAttrs'
+    mkOption
+    nameValuePair
+    removeSuffix
+    types
+    ;
+
+  registryName = fileName: removeSuffix ".nix" fileName;
+
+  registryOption = registry: mkOption {
+    type = types.attrsOf (types.attrsOf registry.type);
+    default = { };
+    description = ''
+      Lazily collected fleet-visible registry entries, nested by host and entry name.
+    '';
+  };
+in
+{
+  load =
+    {
+      dir,
+      args ? { },
+    }:
+    let
+      definitions = mapAttrs' (
+        name: type:
+        let
+          path = dir + "/${name}";
+        in
+        nameValuePair (registryName name) (import path args)
+      ) (filterAttrs (name: type: type == "regular" && hasSuffix ".nix" name) (builtins.readDir dir));
+    in
+    {
+      inherit definitions;
+
+      module = {
+        options.registry = mapAttrs (_name: registry: registryOption registry) definitions;
+      };
+
+      collect =
+        { hosts }:
+        mapAttrs (
+          registryName: _registry:
+          mapAttrs (hostName: host: host.config.registry.${registryName}.${hostName} or { }) hosts
+        ) definitions;
+    };
+}