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/)

phpunit.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         cacheDirectory=".phpunit.cache"
+         failOnWarning="true"
+         failOnRisky="true"
+         failOnEmptyTestSuite="true"
+         beStrictAboutOutputDuringTests="true"
+>
+    <testsuites>
+        <testsuite name="Unit">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <source>
+        <include>
+            <directory>app</directory>
+        </include>
+    </source>
+</phpunit>