fix(single): close remaining live gaps vs Zibll demo

Align runtime --sidebar-width (300→311), --main-radius, and
body.nav-fixed 88px reserve; enqueue single-post CSS parts directly;
match Zibll action buttons and author avatar DOM; tighten dual-column
tokens after comparing blog.rutua.cn with demo.zibll.com.

assets/css/frontend/layout.css

@@ -120,7 +120,7 @@ body.site-layout-1 .content-layout--no-sidebar {
   margin-right: 0 !important;
 }
 
-/* Sidebar RIGHT (layout 2) — Zibll main.css 1:1 */
+/* Sidebar RIGHT (layout 2) — hard 311px like Zibll main.css (runtime var must match) */
 body.site-layout-2 .content-layout {
   margin-right: calc(var(--sidebar-width, 311px) + 15px);
   margin-left: 0;
@@ -130,11 +130,10 @@ body.site-layout-2 .sidebar {
   float: left;
   width: var(--sidebar-width, 311px);
   margin-left: calc(0px - var(--sidebar-width, 311px));
-  /* Zibll uses position:unset so sticky children aren't trapped */
   position: unset;
 }
 
-/* Sidebar LEFT (layout 3) — Zibll main.css 1:1 */
+/* Sidebar LEFT (layout 3) */
 body.site-layout-3 .content-layout {
   margin-left: calc(var(--sidebar-width, 311px) + 15px);
   margin-right: unset;