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/core/scroll.js

@@ -136,28 +136,16 @@
   }
 
   function initNavScrollState() {
-    var nav = document.querySelector(".site-nav");
-
-    var sliderContainer = document.querySelector(".header-components-container");
-    if (nav && sliderContainer) {
-      nav.classList.add("nav-transparent");
-    }
-
-    var scrollDownHide = document.body.classList.contains("scroll-down-hide");
+    // Zibll: hide is CSS on body.scroll-down + header.scroll-down-hide.
+    // Keep is-scrolled on .header for optional consumers.
+    var header = document.querySelector(".header");
     var ticking = false;
 
     function update() {
       ticking = false;
       var body = document.body;
-      if (nav) {
-        nav.classList.toggle("is-scrolled", body.classList.contains("body-scroll"));
-      }
-      if (nav && scrollDownHide) {
-        if (body.classList.contains("scroll-down")) {
-          nav.classList.add("is-hidden");
-        } else {
-          nav.classList.remove("is-hidden");
-        }
+      if (header) {
+        header.classList.toggle("is-scrolled", body.classList.contains("body-scroll"));
       }
     }