✨ v3 theme: glass UI, landing layout, a11y, perf, print styles

Floating glass header with ⌘K search, landing homepage layout with
hero image and GitHub-stars CTA, background canvas with dissolving
heroes, burufugu colour scheme, movie-quote 404, taxonomy pills,
keyboard/ARIA accessibility across menus, search and tabs, idle-
loaded third-party scripts, and dedicated print styles.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

layouts/_default/single.html

@@ -41,7 +41,7 @@
       {{ $enableToc := site.Params.article.showTableOfContents | default false }}
       {{ $enableToc = .Params.showTableOfContents | default $enableToc }}
       {{ $showToc := and $enableToc (in .TableOfContents "<ul") }}
-      {{ $topClass := cond (hasPrefix site.Params.header.layout "fixed") "lg:top-[140px]" "lg:top-10" }}
+      {{ $topClass := cond (or (hasPrefix site.Params.header.layout "fixed") (eq site.Params.header.layout "floating")) "lg:top-[140px]" "lg:top-10" }}
       {{ if $showToc }}
         <div class="order-first lg:ms-auto px-0 lg:order-last lg:ps-8 lg:max-w-2xs">
           <div class="toc ps-5 print:hidden lg:sticky {{ $topClass }}">