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>

.gitignore

@@ -28,5 +28,8 @@ npm-debug.log
 # Claude local settings
 .claude/settings.local.json
 
+# PHPUnit cache
+.phpunit.cache/
+
 # Build artifacts (releases directory)
 releases/