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>

CLAUDE.md

@@ -18,6 +18,7 @@ This project is 100% AI-generated ("vibe-coded") using Claude.AI.
 - **Styling:** Custom CSS
 - **Dependencies:** Composer
 - **i18n:** WordPress i18n (.pot/.po/.mo), text domain: `wc-composable-product`
+- **Testing:** PHPUnit 9.6 + Brain Monkey 2.7 + Mockery 1.6
 - **CI/CD:** Gitea Actions (`.gitea/workflows/release.yml`)
 
 ## Project Structure
@@ -45,10 +46,16 @@ wc-composable-product/
 │   └── StockManager.php       # Stock management & inventory tracking
 ├── languages/                 # Translation files (.pot, .po, .mo)
 ├── releases/                  # Release packages (gitignored)
+├── tests/
+│   ├── bootstrap.php          # Test environment setup (constants, stubs)
+│   ├── TestCase.php           # Base test case with Brain Monkey
+│   ├── stubs/                 # Minimal WooCommerce class stubs
+│   └── Unit/                  # PHPUnit unit tests
 ├── templates/
 │   └── product-selector.twig  # Frontend selection interface
 ├── vendor/                    # Composer dependencies (gitignored, included in releases)
 ├── composer.json
+├── phpunit.xml.dist           # PHPUnit configuration
 └── wc-composable-product.php  # Main plugin file
 ```
 
@@ -133,11 +140,17 @@ Compile .po to .mo: `for po in languages/*.po; do msgfmt -o "${po%.po}.mo" "$po"
 
 WordPress requires compiled .mo files — .po files alone are insufficient.
 
+## Testing
+
+Run unit tests: `vendor/bin/phpunit --testdox`
+
+Tests use **Brain Monkey** to mock WordPress/WooCommerce functions without a full WP installation. WooCommerce classes (`WC_Product`, `WC_Settings_Page`, etc.) are provided as minimal stubs in `tests/stubs/` so PHP can parse `extends` declarations. The release workflow runs tests before building — a failing test blocks the release.
+
 ## Release Workflow
 
 ### Automated (Gitea CI/CD)
 
-Push an annotated tag (`v*`) to trigger the workflow. It installs PHP 8.3, production Composer deps, compiles translations, verifies version matches tag, creates ZIP with checksums, and publishes a Gitea release.
+Push an annotated tag (`v*`) to trigger the workflow. It first runs the PHPUnit test suite, then installs PHP 8.3, production Composer deps, compiles translations, verifies version matches tag, creates ZIP with checksums, and publishes a Gitea release. Tests must pass before the release package is built.
 
 ### Manual