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>

README.md

@@ -12,7 +12,8 @@ Create composable products where customers can select a limited number of items
 - **Pricing Options**: Fixed price or sum of selected products with full locale-aware formatting
 - **Multi-language Support**: Fully translated in 6 locales (de_DE, de_CH, fr_CH, it_CH + informal variants)
 - **Modern UI**: Clean interface built with Twig templates and vanilla JavaScript
-- **CI/CD**: Automated release workflow for Gitea
+- **Tested**: 57 unit tests with PHPUnit, Brain Monkey, and Mockery
+- **CI/CD**: Automated release workflow with test gate for Gitea
 
 ## Requirements
 
@@ -118,6 +119,16 @@ This project was created with AI assistance (Claude.AI) and follows WordPress an
 composer install
 ```
 
+### Running Tests
+
+The plugin includes a PHPUnit test suite with Brain Monkey for WordPress function mocking:
+
+```bash
+vendor/bin/phpunit --testdox
+```
+
+Tests run without a WordPress installation. WooCommerce classes are provided as minimal stubs in `tests/stubs/`.
+
 ### Translation
 
 Generate POT file: