Marsh demo: video embed with reactive+hypermedia interplay

- ~video-player defisland persists across SPA navigations (morph-safe)
- Clicking "reactive" cycles colour (signal) + fetches random YouTube video (sx-get)
- sx-trigger="fetch-video" + dom-first-child check: video keeps playing on repeat clicks
- Close button (x) clears video via /api/clear-video hypermedia endpoint
- Autoplay+mute removes YouTube's red play button overlay
- Header restructured: logo in anchor, tagline outside (no accidental navigation)
- Flex centering on video container

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sxc/home.sx

@@ -1,5 +1,19 @@
 ;; SX docs — home page components
 
+;; YouTube embed — rendered client-side from video ID returned by /api/random-video.
+;; Marsh demo: the server picks the video (hypermedia), the island triggers the fetch (reactive).
+(defcomp ~video-embed (&key video-id)
+  (<>
+    (button
+      :sx-get "/api/clear-video" :sx-target "#video-embed"
+      :sx-swap "innerHTML"
+      :style "position:absolute;top:-0.5rem;right:-0.5rem;width:1.25rem;height:1.25rem;border-radius:50%;border:none;background:rgba(0,0,0,0.5);color:white;font-size:0.75rem;line-height:1;cursor:pointer;display:flex;align-items:center;justify-content:center;z-index:10;"
+      "x")
+    (iframe :src (str "https://www.youtube.com/embed/" video-id "?autoplay=1&mute=1")
+      :allow "accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
+      :allowfullscreen "true"
+      :style "width:100%;aspect-ratio:16/9;border-radius:0.5rem;border:none;")))
+
 (defcomp ~sx-hero (&key &rest children)
   (div :class "max-w-4xl mx-auto px-6 py-16 text-center"
     (h1 :class "text-5xl font-bold text-stone-900 mb-4"