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>

phpcs.xml.dist

@@ -0,0 +1,34 @@
+<?xml version="1.0"?>
+<ruleset name="WC Composable Product">
+    <description>PHPCS rules for WooCommerce Composable Products plugin</description>
+
+    <!-- Scan plugin source -->
+    <file>includes</file>
+    <file>wc-composable-product.php</file>
+
+    <!-- Show progress and use colors -->
+    <arg value="ps"/>
+    <arg name="colors"/>
+
+    <!-- Use WordPress Extra standard (Core + Extra, without Docs) -->
+    <rule ref="WordPress-Extra">
+        <!-- Allow PSR-4 PascalCase file naming -->
+        <exclude name="WordPress.Files.FileName"/>
+    </rule>
+
+    <!-- Check PHP 8.3+ compatibility -->
+    <rule ref="PHPCompatibilityWP"/>
+    <config name="testVersion" value="8.3-"/>
+
+    <!-- WordPress minimum version -->
+    <config name="minimum_wp_version" value="6.0"/>
+
+    <!-- Enforce text domain -->
+    <rule ref="WordPress.WP.I18n">
+        <properties>
+            <property name="text_domain" type="array">
+                <element value="wc-composable-product"/>
+            </property>
+        </properties>
+    </rule>
+</ruleset>