fix(single): restore Zibll float dual-column and article chrome

Align single-post layout/visuals with Zibll: fix sidebar-over-content
float geometry (position:unset, 311px token, kill Bootstrap/container
padding and the 280px sidebar override), demote theme-box to spacing-
only, unify breadcrumb/cover/graphic utilities, and split aksrt-single
render helpers under the file-size limit.

assets/css/frontend/single-post/shell.css

@@ -25,7 +25,7 @@ body.single main.container .content-layout > .breadcrumbs-wrap {
   padding-top: 2px;
 }
 
-/* Sticky TOC widget in sidebar */
+/* Sticky TOC widget in sidebar — parent .sidebar is position:unset like Zibll */
 body.single .sidebar .posts-nav-box.toc-widget {
   position: sticky;
   top: calc(var(--nav-height, 60px) + 16px);
@@ -35,3 +35,8 @@ body.single .sidebar .zib-widget,
 body.single .sidebar .widget {
   margin-bottom: 15px;
 }
+
+/* Geometry owned by base/layout — keep single shell free of float overrides. */
+body.single main.container {
+  box-sizing: border-box;
+}