feat(registry): add generic fleet namespace

lib/registry.nix

@@ -0,0 +1,45 @@
+{ lib, self }:
+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.
+    '';
+  };
+
+  definitions = mapAttrs' (
+    name: type:
+    let
+      path = self + "/registry/${name}";
+    in
+    nameValuePair (registryName name) (import path { inherit lib; })
+  ) (filterAttrs (name: type: type == "regular" && hasSuffix ".nix" name) (builtins.readDir (self + "/registry")));
+in
+{
+  inherit definitions;
+
+  module = {
+    options.registry = mapAttrs (_name: registry: registryOption registry) definitions;
+  };
+
+  collect =
+    { hosts }:
+    mapAttrs (
+      registryName: _registry:
+      mapAttrs (hostName: host: host.config.registry.${registryName}.${hostName} or { }) hosts
+    ) definitions;
+}