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>

shared/sx/ref/platform_js.py

@@ -1659,12 +1659,20 @@ PLATFORM_DOM_JS = """
   function domCallMethod() {
     var obj = arguments[0], method = arguments[1];
     var args = Array.prototype.slice.call(arguments, 2);
-    console.log("[sx] dom-call-method:", obj, method, args);
     if (obj && typeof obj[method] === 'function') {
       try { return obj[method].apply(obj, args); }
       catch(e) { console.error("[sx] dom-call-method error:", e); return NIL; }
     }
-    console.warn("[sx] dom-call-method: method not found or obj null", obj, method);
+    return NIL;
+  }
+  // Post a message to an iframe's contentWindow without exposing the cross-origin
+  // Window object to the SX evaluator (which would trigger _thunk access errors).
+  function domPostMessage(iframe, msg, origin) {
+    try {
+      if (iframe && iframe.contentWindow) {
+        iframe.contentWindow.postMessage(msg, origin || '*');
+      }
+    } catch(e) { console.error("[sx] domPostMessage error:", e); }
     return NIL;
   }
 
@@ -2886,6 +2894,7 @@ def fixups_js(has_html, has_sx, has_dom, has_signals=False, has_deps=False, has_
   PRIMITIVES["dom-get-prop"] = domGetProp;
   PRIMITIVES["dom-set-prop"] = domSetProp;
   PRIMITIVES["dom-call-method"] = domCallMethod;
+  PRIMITIVES["dom-post-message"] = domPostMessage;
   PRIMITIVES["stop-propagation"] = stopPropagation_;
   PRIMITIVES["error-message"] = errorMessage;
   PRIMITIVES["schedule-idle"] = scheduleIdle;