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>

CHANGELOG.md

@@ -2,6 +2,16 @@
 
 All notable changes to this project will be documented in this file.
 
+## [0.1.6] - 2026-03-01
+
+### Added
+
+- **PHPUnit test suite** with Brain\Monkey for WordPress function mocking (27 tests, 54 assertions)
+- **TemplateOverrideTest**: Tests for hook registration, Twig template resolution, output buffer stack (including nesting), context passing, global `$product` injection, and exception fallback
+- **WooCommerceExtensionTest**: Tests for function/filter registration, `callFunction()` whitelist enforcement, `doAction`/`applyFilters`/`callUserFunc` output capture, `setupProductData`, and all output-capture wrappers
+- **CI/CD test job**: PHPUnit runs between lint and build-release in Gitea pipeline; test artifacts excluded from release packages
+- Class stubs for `WPBootstrap\Twig\TwigService` and `WC_Product` enabling isolated unit testing without WordPress/WooCommerce
+
 ## [0.1.5] - 2026-03-01
 
 ### Fixed