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>

.gitignore

@@ -35,3 +35,4 @@ releases/
 # Docker runtime
 .env
 KNOWN_BUGS.md
+.phpunit.cache/