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/Core/View.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Core;
+
+class View
+{
+    private string $viewsPath;
+    private array $data = [];
+
+    public function __construct(?string $viewsPath = null)
+    {
+        $this->viewsPath = $viewsPath ?? dirname(__DIR__) . '/views';
+    }
+
+    public function assign(string $key, mixed $value): void
+    {
+        $this->data[$key] = $value;
+    }
+
+    public function render(string $view, string $layout = 'main'): void
+    {
+        $viewFile = $this->viewsPath . '/' . $view . '.php';
+        $layoutFile = $this->viewsPath . '/layouts/' . $layout . '.php';
+
+        if (!file_exists($viewFile)) {
+            throw new \RuntimeException("View {$view} nicht gefunden: {$viewFile}");
+        }
+
+        if (!file_exists($layoutFile)) {
+            throw new \RuntimeException("Layout {$layout} nicht gefunden: {$layoutFile}");
+        }
+
+        // Extract data to variables for the view
+        extract($this->data, EXTR_SKIP);
+
+        // Capture view content
+        ob_start();
+        require $viewFile;
+        $content = ob_get_clean();
+
+        // Render layout with $content
+        require $layoutFile;
+    }
+}