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

phpunit.dist.xml

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!-- https://phpunit.readthedocs.io/en/latest/configuration.html -->
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         colors="true"
+         failOnDeprecation="true"
+         failOnNotice="true"
+         failOnWarning="true"
+         bootstrap="tests/bootstrap.php"
+         cacheDirectory=".phpunit.cache"
+>
+    <php>
+        <ini name="display_errors" value="1" />
+        <ini name="error_reporting" value="-1" />
+        <server name="APP_ENV" value="test" force="true" />
+        <server name="SHELL_VERBOSITY" value="-1" />
+    </php>
+
+    <testsuites>
+        <testsuite name="Project Test Suite">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <source ignoreSuppressionOfDeprecations="true"
+            ignoreIndirectDeprecations="true"
+            restrictNotices="true"
+            restrictWarnings="true"
+    >
+        <include>
+            <directory>src</directory>
+        </include>
+
+        <deprecationTrigger>
+            <function>trigger_deprecation</function>
+        </deprecationTrigger>
+    </source>
+
+    <extensions>
+    </extensions>
+</phpunit>