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>

.gitea/workflows/release.yml

@@ -6,7 +6,52 @@ on:
       - 'v*'
 
 jobs:
+  lint:
+    name: PHP Lint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+          fetch-depth: 0
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.3'
+          extensions: mbstring, xml, zip
+          tools: composer:v2
+
+      - name: PHP Syntax Check
+        run: |
+          find includes -name "*.php" -print0 | xargs -0 -n1 php -l
+          find tests -name "*.php" -print0 | xargs -0 -n1 php -l
+
+  test:
+    runs-on: ubuntu-latest
+    needs: lint
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.3'
+          extensions: mbstring, xml
+          tools: composer:v2
+
+      - name: Install Composer dependencies
+        run: |
+          composer config platform.php 8.3.0
+          composer install --optimize-autoloader --no-interaction
+
+      - name: Run PHPUnit tests
+        run: vendor/bin/phpunit --testdox
+
   build-release:
+    needs: test
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code