Security audit fixes: regex hardening, performance, and code quality (v1.1.2)

- WidgetRenderer: single regex for h2→h4 prevents mismatched tags
- ContextBuilder: O(n) comment tree with parent-indexed lookup map
- ContextBuilder: consolidated sidebar queries into single check
- ContextBuilder: transient caching for sidebar recent posts and tags
- functions.php: hex-to-RGB consolidation, type hints, ctype_xdigit validation
- Transient invalidation hooks for save_post and tag CRUD operations

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

style.css

@@ -7,7 +7,7 @@ Description: A modern WordPress Block Theme built from scratch with Bootstrap 5.
 Requires at least: 6.7
 Tested up to: 6.7
 Requires PHP: 8.3
-Version: 1.1.1
+Version: 1.1.2
 License: GNU General Public License v2 or later
 License URI: http://www.gnu.org/licenses/gpl-2.0.html
 Text Domain: wp-bootstrap