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>

tests/Unit/PluginTest.php

@@ -0,0 +1,72 @@
+<?php
+/**
+ * Plugin Tests
+ *
+ * @package Magdev\WcComposableProduct\Tests
+ */
+
+namespace Magdev\WcComposableProduct\Tests\Unit;
+
+use Magdev\WcComposableProduct\Tests\TestCase;
+use Magdev\WcComposableProduct\Plugin;
+use Brain\Monkey\Functions;
+
+class PluginTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        // Reset the singleton instance between tests
+        $reflection = new \ReflectionClass(Plugin::class);
+        $property = $reflection->getProperty('instance');
+        $property->setAccessible(true);
+        $property->setValue(null, null);
+    }
+
+    public function testInstance_ReturnsSingleton(): void
+    {
+        $instance1 = Plugin::instance();
+        $instance2 = Plugin::instance();
+
+        $this->assertSame($instance1, $instance2);
+    }
+
+    public function testInstance_ReturnsPluginClass(): void
+    {
+        $instance = Plugin::instance();
+
+        $this->assertInstanceOf(Plugin::class, $instance);
+    }
+
+    public function testAddProductType_AddsComposableToTypes(): void
+    {
+        $plugin = Plugin::instance();
+        $types = $plugin->add_product_type([]);
+
+        $this->assertArrayHasKey('composable', $types);
+    }
+
+    public function testProductClass_ReturnsCustomClassForComposable(): void
+    {
+        $plugin = Plugin::instance();
+        $class = $plugin->product_class('WC_Product', 'composable');
+
+        $this->assertSame('Magdev\WcComposableProduct\ProductType', $class);
+    }
+
+    public function testProductClass_PassesThroughForOtherTypes(): void
+    {
+        $plugin = Plugin::instance();
+        $class = $plugin->product_class('WC_Product', 'simple');
+
+        $this->assertSame('WC_Product', $class);
+    }
+
+    public function testGetTwig_ReturnsTwigEnvironment(): void
+    {
+        $plugin = Plugin::instance();
+
+        $this->assertInstanceOf(\Twig\Environment::class, $plugin->get_twig());
+    }
+}