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/TwigService.php

@@ -0,0 +1,53 @@
+<?php
+/**
+ * Stub for WPBootstrap\Twig\TwigService.
+ *
+ * Provides just enough surface for TemplateOverride to resolve
+ * and render templates during unit tests.
+ */
+
+namespace WPBootstrap\Twig;
+
+class TwigService
+{
+    private static ?self $instance = null;
+
+    /** @var callable|null Render callback set by tests. */
+    private static $renderCallback = null;
+
+    public static function getInstance(): self
+    {
+        if (null === self::$instance) {
+            self::$instance = new self();
+        }
+        return self::$instance;
+    }
+
+    /**
+     * Allow tests to override the render behaviour.
+     */
+    public static function setRenderCallback(?callable $callback): void
+    {
+        self::$renderCallback = $callback;
+    }
+
+    /**
+     * Render a template with the given context.
+     */
+    public function render(string $template, array $context = []): string
+    {
+        if (null !== self::$renderCallback) {
+            return (self::$renderCallback)($template, $context);
+        }
+        return '';
+    }
+
+    /**
+     * Reset singleton between tests.
+     */
+    public static function reset(): void
+    {
+        self::$instance = null;
+        self::$renderCallback = null;
+    }
+}