feat(wpmc S3): ShortcodeDmr scaffold + register init hook

tests/ShortcodeDmrTest.php

@@ -0,0 +1,19 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity\Tests;
+
+use WPMultiCity\ShortcodeDmr;
+
+final class ShortcodeDmrTest extends TestCase
+{
+    public function test_register_attaches_init_action(): void
+    {
+        ShortcodeDmr::register();
+
+        self::assertNotFalse(
+            has_action('init', [ShortcodeDmr::class, 'do_register']),
+            'ShortcodeDmr::register() must hook init'
+        );
+    }
+}