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>

composer.json

@@ -18,6 +18,16 @@
             "Magdev\\WcComposableProduct\\": "includes/"
         }
     },
+    "require-dev": {
+        "brain/monkey": "^2.7",
+        "mockery/mockery": "^1.6",
+        "phpunit/phpunit": "^9.6"
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Magdev\\WcComposableProduct\\Tests\\": "tests/"
+        }
+    },
     "config": {
         "optimize-autoloader": true,
         "sort-packages": true