Add PHPUnit test suite with Brain\Monkey (v0.1.6)

Add test infrastructure for isolated unit testing without WordPress/WooCommerce:
- 27 tests (54 assertions) covering TemplateOverride and WooCommerceExtension
- Brain\Monkey for WordPress function mocking, class stubs for TwigService and WC_Product
- PHPUnit test job added to Gitea CI pipeline between lint and build-release
- Test artifacts excluded from release packages

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

composer.json

@@ -14,11 +14,20 @@
         "php": ">=8.3",
         "twig/twig": "^3.0"
     },
+    "require-dev": {
+        "brain/monkey": "^2.6",
+        "phpunit/phpunit": "^11.0"
+    },
     "autoload": {
         "psr-4": {
             "WcBootstrap\\": "inc/"
         }
     },
+    "autoload-dev": {
+        "psr-4": {
+            "WcBootstrap\\Tests\\": "tests/"
+        }
+    },
     "config": {
         "optimize-autoloader": true,
         "sort-packages": true