IO registry: defio declares platform suspension points

Core SX has zero IO — platforms extend __io-registry via (defio name
:category :data/:code/:effect ...). The server web platform declares 44
operations in web/io.sx. batchable_helpers now derived from registry
(:batchable true) instead of hardcoded list. Startup validation warns if
bound IO ops lack registry entries. Browser gets empty registry, ready
for step 5 (IO suspension).

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

hosts/ocaml/browser/sx_browser.ml

@@ -671,6 +671,27 @@ let () =
 
   bind "define-page-helper" (fun _ -> Nil);
 
+  (* IO registry — starts empty in browser. Platforms extend via defio.
+     Browser has zero suspension points initially; future browser IO
+     (lazy module loads, fetch-request) will add entries here. *)
+  let io_registry = Hashtbl.create 16 in
+  ignore (env_bind global_env "__io-registry" (Dict io_registry));
+  ignore (Sx_ref.register_special_form (String "defio") (NativeFn ("defio", fun sf_args ->
+    let raw_args = match sf_args with
+      | [List a; Env _] | [ListRef { contents = a }; Env _] -> a
+      | _ -> [] in
+    match raw_args with
+    | String name :: rest ->
+      let entry = Hashtbl.create 8 in
+      let rec parse = function
+        | Keyword k :: v :: rest -> Hashtbl.replace entry k v; parse rest
+        | _ -> () in
+      parse rest;
+      Hashtbl.replace entry "name" (String name);
+      Hashtbl.replace io_registry name (Dict entry);
+      Dict entry
+    | _ -> Nil)));
+
   (* --- Render --- *)
   Sx_render.setup_render_env global_env;
   bind "set-render-active!" (fun _ -> Nil);