Stepper cookie persistence: SSR + client-side save/restore

- Parse Cookie header in OCaml HTTP server for get-cookie primitive
- Stepper saves step-idx to cookie via host-set! FFI on click
- Stepper restores from cookie: get-cookie on server, host-get FFI on client
- Cache key includes stepper cookie value to avoid stale SSR
- registerNative: also update Sx_primitives table for CALL_PRIM dispatch

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/browser/sx_browser.ml

@@ -401,6 +401,7 @@ let api_register_native name_js callback_js =
     js_to_value (Js.Unsafe.fun_call callback_js [| Js.Unsafe.inject (Js.array js_args) |])
   in
   let v = NativeFn (name, native_fn) in
+  Sx_primitives.register name native_fn;
   ignore (env_bind global_env name v);
   Hashtbl.replace _vm_globals name v;
   Js.Unsafe.inject Js.null