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>

tests/bootstrap.php

@@ -0,0 +1,14 @@
+<?php
+/**
+ * PHPUnit bootstrap file.
+ *
+ * Loads Composer autoloader and class stubs required
+ * for unit testing outside of WordPress/WooCommerce.
+ */
+
+// Composer autoloader (loads WcBootstrap\* classes + Brain\Monkey).
+require_once dirname(__DIR__) . '/vendor/autoload.php';
+
+// Class stubs (WordPress / WooCommerce / parent theme).
+require_once __DIR__ . '/Stubs/TwigService.php';
+require_once __DIR__ . '/Stubs/WcProduct.php';