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>

.gitignore

@@ -25,3 +25,4 @@ phpunit.xml
 languages/*.mo
 
 .phpunit.result.cache
+.phpunit.cache/