Add PHPUnit test suite with 64 unit tests (v1.1.1)

PHPUnit 11 + Brain\Monkey for WordPress function mocking. Tests cover
BlockRenderer (28), WidgetRenderer (9), NavWalker (14), and
TemplateController (12). Includes functional WP_HTML_Tag_Processor stub,
CI test job between lint and build-release, prebuild hook gating npm
build on passing tests, and release package exclusions for test files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/Stubs/WpWidget.php

@@ -0,0 +1,10 @@
+<?php
+/**
+ * Stub for WordPress WP_Widget class.
+ *
+ * Used as a type hint in WidgetRenderer::processBlockWidgetContent().
+ * Only needs to exist — no functionality required.
+ */
+class WP_Widget
+{
+}