feat: add PHPUnit test infrastructure and Router tests

- Add composer.json with PHPUnit 11 and PSR-4 autoloading
- Add phpunit.xml configuration
- Rename app/core/ → app/Core/ and app/controllers/ → app/Controllers/ (PSR-4)
- Add 18 unit tests for App\Core\Router (31 assertions)
  - addRoute(): default action, custom action, overwrite
  - dispatch(): URL normalization, direct match, legacy redirects
  - dispatch(): 404 handling, controller/action not found exceptions
  - TestableRouter subclass to intercept side-effects
- Update .gitignore (vendor/, .phpunit.cache/)

app/Controllers/ImpressumController.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Controllers;
+
+class ImpressumController extends Controller
+{
+    public function index(): void
+    {
+        $this->render('impressum/index', [
+            'pageTitle'       => 'Impressum – Haus Schleusingen',
+            'pageDescription' => 'Impressum der Website haus-schleusingen.de',
+            'robots'          => 'noindex',
+            'canonical'       => 'https://haus-schleusingen.de/impressum',
+        ]);
+    }
+}