IO proxy: client-side cache with 5min TTL, server Cache-Control

Client caches IO results by (name + args) in memory. In-flight
promises are cached too (dedup concurrent calls for same args).
Server adds Cache-Control: public, max-age=300 for HTTP caching.

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

shared/sx/pages.py

@@ -601,6 +601,7 @@ def mount_io_endpoint(app: Any, service_name: str) -> None:
         result_sx = serialize(result) if result is not None else "nil"
         resp = await make_response(result_sx, 200)
         resp.content_type = "text/sx; charset=utf-8"
+        resp.headers["Cache-Control"] = "public, max-age=300"
         return resp
 
     io_proxy.__name__ = "sx_io_proxy"