podman: add darwin support with machine management

- restructure module from `podman-linux` to platform-agnostic `podman`
- move linux-specific implementation to `modules/services/podman/linux/`
- add darwin module with declarative machine management
- implement launchd-based watchdog for auto-starting machines
- maintains backward compatibility with existing linux functionality

modules/services/podman/linux/options.nix

@@ -0,0 +1,76 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  inherit (lib) mkEnableOption mkIf mkOption;
+  assertions = import ../assertions.nix { inherit lib; };
+
+  cfg = config.services.podman;
+
+  # Define the systemd service type
+  quadletInternalType = lib.types.submodule {
+    options = {
+      assertions = mkOption {
+        type = with lib.types; listOf unspecified;
+        default = [ ];
+        internal = true;
+        description = "List of Nix type assertions.";
+      };
+
+      dependencies = mkOption {
+        type = with lib.types; listOf package;
+        default = [ ];
+        internal = true;
+        description = "List of systemd service dependencies.";
+      };
+
+      resourceType = mkOption {
+        type = lib.types.str;
+        default = "";
+        internal = true;
+        description = "The type of the podman Quadlet resource.";
+      };
+
+      serviceName = mkOption {
+        type = lib.types.str;
+        internal = true;
+        description = "The name of the systemd service.";
+      };
+
+      source = mkOption {
+        type = lib.types.str;
+        internal = true;
+        description = "The quadlet source file content.";
+      };
+    };
+  };
+in
+{
+  options.services.podman = {
+    internal = {
+      quadletDefinitions = mkOption {
+        type = lib.types.listOf quadletInternalType;
+        default = { };
+        internal = true;
+        description = "List of quadlet source file content and service names.";
+      };
+      builtQuadlets = mkOption {
+        type = with lib.types; attrsOf package;
+        default = { };
+        internal = true;
+        description = "All built quadlets.";
+      };
+    };
+
+    enableTypeChecks = mkEnableOption "type checks for podman quadlets";
+  };
+  config = mkIf cfg.enable {
+    assertions = [
+      (assertions.assertPlatform "services.podman.enableTypeChecks" config pkgs lib.platforms.linux)
+    ];
+  };
+}