Add DOM primitives (dom-set-prop, dom-call-method, dom-post-message), bump SW cache v2, remove video demo

New platform_js primitives for direct DOM property/method access and
cross-origin iframe communication. Service worker static cache bumped
to v2 to flush stale assets. Removed experimental video embed from
header island, routes, and home page.

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

sx/sx/docs-content.sx

@@ -2,9 +2,7 @@
 
 (defcomp ~sx-home-content ()
   (div :id "main-content" :class "max-w-3xl mx-auto px-4 py-6"
-    (~doc-code :code (highlight (component-source "~sx-header") "lisp"))
-    (~doc-code :code (highlight (component-source "~video-player") "lisp"))
-    (~doc-code :code (highlight (component-source "~video-embed") "lisp"))))
+    (~doc-code :code (highlight (component-source "~sx-header") "lisp"))))
 
 (defcomp ~docs-introduction-content ()
   (~doc-page :title "Introduction"