Add PHPUnit test suite, PSR-4 refactor, lint+test CI jobs (v1.3.1)

- 57 unit tests covering ProductType, StockManager, CartHandler, Plugin,
  Admin/ProductData, Admin/Settings using Brain Monkey + Mockery
- WooCommerce class stubs for testing without WP installation
- PHP lint and test jobs in release workflow (test gate blocks release)
- PSR-4 namespace change: WC_Composable_Product -> Magdev\WcComposableProduct
- PascalCase filenames for all classes under includes/

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

phpunit.xml.dist

@@ -0,0 +1,24 @@
+<?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"
+    bootstrap="tests/bootstrap.php"
+    colors="true"
+    verbose="true"
+    beStrictAboutTestsThatDoNotTestAnything="true"
+    beStrictAboutOutputDuringTests="true"
+    failOnRisky="false"
+    failOnWarning="true"
+>
+    <testsuites>
+        <testsuite name="Unit">
+            <directory suffix="Test.php">tests/Unit</directory>
+        </testsuite>
+    </testsuites>
+
+    <coverage processUncoveredFiles="true">
+        <include>
+            <directory suffix=".php">includes</directory>
+        </include>
+    </coverage>
+</phpunit>