feat(nav): rewrite top header to zibll 1:1 DOM/CSS/JS contract

Replace hybrid site-nav/drawer shell with zibll navbar-top layouts 1/2/3,
body.mobile-navbar-show drawer, wired NavWalker + aksrt_menu_options meta,
nav_folding/chevron, VIP switches, and SPA/PHP schema parity. No dual-class
compat paths.

assets/js/aksrt-runtime.js

@@ -276,7 +276,7 @@
       top = target.getBoundingClientRect().top + window.pageYOffset;
     }
 
-    var navFixed = document.querySelector('.site-nav.is-fixed, .site-header.is-fixed');
+    var navFixed = document.querySelector('body.nav-fixed .header, .navbar.navbar-top');
     var headerOffset = navFixed ? (navFixed.offsetHeight + 20) : 0;
     top = Math.max(0, top - headerOffset - (extraOffset || 0));