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

tests/modules/services/podman/linux/image.nix

@@ -0,0 +1,24 @@
+{ config, lib, ... }:
+{
+  imports = [ ./podman-stubs.nix ];
+  config = lib.mkIf config.test.enableLegacyIfd {
+    services.podman = {
+      enable = true;
+      images = {
+        "my-img" = {
+          image = "docker.io/alpine:latest";
+        };
+      };
+    };
+
+    nmt.script = ''
+      configPath=home-files/.config/systemd/user
+      imageFile=$configPath/podman-my-img-image.service
+      assertFileExists $imageFile
+
+      imageFile=$(normalizeStorePaths $imageFile)
+
+      assertFileContent $imageFile ${./image-expected.service}
+    '';
+  };
+}