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/bp/pages/routes.py

@@ -1049,23 +1049,6 @@ def register(url_prefix: str = "/") -> Blueprint:
         resp.headers["SX-Retarget"] = "#ref-hdr-retarget-alt"
         return resp
 
-    # --- Random video (marsh demo: reactive click + hypermedia fetch) ---
-
-    _last_video = {"id": ""}
-
-    @bp.get("/api/random-video")
-    async def api_random_video():
-        from shared.sx.helpers import sx_response
-        videos = ["mSil_hBqbac", "-EX4mgvnSc8", "maOM81a-Gyg", "OHPJemGxUjE"]
-        available = [v for v in videos if v != _last_video["id"]]
-        video_id = random.choice(available)
-        _last_video["id"] = video_id
-        return sx_response(f'(~video-embed :video-id "{video_id}")')
-
-    @bp.get("/api/clear-video")
-    async def api_clear_video():
-        return Response("", content_type="text/html")
-
     # --- Event demos ---
 
     @bp.get("/geography/hypermedia/reference/api/error-500")