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>

CHANGELOG.md

@@ -5,6 +5,23 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.3.1] - 2026-03-01
+
+### Added
+
+- **PHPUnit test suite**: 57 unit tests covering all 6 core classes (ProductType, StockManager, CartHandler, Plugin, Admin/ProductData, Admin/Settings)
+- **Brain Monkey + Mockery** for WordPress/WooCommerce function mocking without a full WP installation
+- **WooCommerce class stubs** in `tests/stubs/` for classes extended by the plugin (WC_Product, WC_Settings_Page, etc.)
+- **PHP lint job** in release workflow — syntax-checks all PHP files before testing
+- **Test job** in release workflow — tests must pass before release package is built (`needs: test`)
+- Testing section in README
+
+### Changed
+
+- **PSR-4 refactored**: Renamed files to PascalCase (Product_Type → ProductType, etc.) and changed namespace from `WC_Composable_Product` to `Magdev\WcComposableProduct`
+- Updated all cross-references in PHP files, main plugin file, composer.json, CSS/JS doc comments, and translation file source comments
+- Release workflow now has three stages: lint → test → build-release
+
 ## [1.3.0] - 2026-03-01
 
 ### Added