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>

composer.json

@@ -20,8 +20,12 @@
     },
     "require-dev": {
         "brain/monkey": "^2.7",
+        "dealerdirect/phpcodesniffer-composer-installer": "^1.0",
         "mockery/mockery": "^1.6",
-        "phpunit/phpunit": "^9.6"
+        "phpcompatibility/phpcompatibility-wp": "*",
+        "phpunit/phpunit": "^10.0",
+        "squizlabs/php_codesniffer": "^3.7",
+        "wp-coding-standards/wpcs": "^3.0"
     },
     "autoload-dev": {
         "psr-4": {
@@ -30,6 +34,12 @@
     },
     "config": {
         "optimize-autoloader": true,
-        "sort-packages": true
+        "sort-packages": true,
+        "allow-plugins": {
+            "dealerdirect/phpcodesniffer-composer-installer": true
+        },
+        "platform": {
+            "php": "8.3.0"
+        }
     }
 }