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/Stubs/WcProduct.php

@@ -0,0 +1,21 @@
+<?php
+/**
+ * Stub for WC_Product.
+ *
+ * Minimal implementation for type-hint satisfaction in unit tests.
+ */
+
+class WC_Product
+{
+    private int $id;
+
+    public function __construct(int $id = 0)
+    {
+        $this->id = $id;
+    }
+
+    public function get_id(): int
+    {
+        return $this->id;
+    }
+}