Add PHPUnit test suite and supporting infrastructure

- Add phpunit/phpunit ^13.2, symfony/browser-kit and symfony/css-selector
  to require-dev, plus the autoload-dev mapping for App\Tests- Add phpunit.dist.xml (strict deprecation/notice/warning failures,
  APP_ENV=test forced) and .env.test / bin/phpunit / tests/bootstrap.php
  from the PHPUnit recipe
- Add tests/Support/TotpTestHelper providing a deterministic TOTP
  fixture, frozen clock and ConfigBag/cache-pool helpers
- Add 121 unit tests covering Clock, ConfigBag, Data/Payload, Enum/Scope,
  MonitorCacheKeys, PersistCache, Utilities, all five Traits and the
  three Service managers (BackupCode, Domain, Login)
- Fix LoginManagerTest nonce lookups to use makeCacheKey() so the cache
  key matches the one the manager actually reads/writes
- Gitignore bin/.phpunit.result.cache

tests/Unit/ClockTest.php

@@ -0,0 +1,20 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Tests\Unit;
+
+use App\Clock;
+use PHPUnit\Framework\TestCase;
+
+final class ClockTest extends TestCase {
+    public function testNowReturnsDateTimeImmutable(): void {
+        $clock = new Clock();
+        $before = new \DateTimeImmutable();
+        $now = $clock->now();
+        $after = new \DateTimeImmutable();
+
+        self::assertInstanceOf(\DateTimeImmutable::class, $now);
+        self::assertGreaterThanOrEqual($before->getTimestamp(), $now->getTimestamp());
+        self::assertLessThanOrEqual($after->getTimestamp(), $now->getTimestamp());
+    }
+}