Upgrade to PHPUnit 10, add PHPCS with WPCS compliance, add phpcs CI job

- Upgrade PHPUnit 9.6 → 10, update phpunit.xml.dist schema
- Add PHPCS 3.13 with WordPress-Extra + PHPCompatibilityWP standards
- PHPCBF auto-fix + manual fixes for full WPCS compliance
- Add phpcs job to release workflow (parallel with lint)
- Pin composer platform to PHP 8.3 to prevent incompatible dep locks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

phpunit.xml.dist

@@ -1,14 +1,11 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <phpunit
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.6/phpunit.xsd"
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.0/phpunit.xsd"
     bootstrap="tests/bootstrap.php"
     colors="true"
-    verbose="true"
-    beStrictAboutTestsThatDoNotTestAnything="true"
-    beStrictAboutOutputDuringTests="true"
-    failOnRisky="false"
     failOnWarning="true"
+    cacheDirectory=".phpunit.cache"
 >
     <testsuites>
         <testsuite name="Unit">
@@ -16,9 +13,9 @@
         </testsuite>
     </testsuites>
 
-    <coverage processUncoveredFiles="true">
+    <source>
         <include>
             <directory suffix=".php">includes</directory>
         </include>
-    </coverage>
+    </source>
 </phpunit>