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/Controller.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Controllers;
+
+use App\Core\View;
+
+abstract class Controller
+{
+    protected View $view;
+
+    public function __construct()
+    {
+        $this->view = new View();
+    }
+
+    protected function render(string $view, array $data = [], string $layout = 'main'): void
+    {
+        foreach ($data as $key => $value) {
+            $this->view->assign($key, $value);
+        }
+        $this->view->render($view, $layout);
+    }
+}