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/enhancements.js

@@ -525,7 +525,7 @@
    * 菜单项超出容器宽度时折叠进「更多」下拉
    * ============================================================ */
   function initNavOverflow() {
-    var nav = document.querySelector('.site-nav .nav-menu, .site-nav .primary-nav, .header-nav__menu');
+    var nav = document.querySelector('.navbar-top .nav.navbar-nav');
     if (!nav) return;
 
     // 创建「更多」按钮