IO suspension driver: _driveAsync in platform, VmSuspended in value_to_js

- sx-platform.js: add _driveAsync to platform (was sandbox-only) for
  driving wait/fetch IO suspension chains in live site
- sx-platform.js: host-callback wrapper calls _driveAsync on callFn result
- sx_browser.ml: value_to_js callable wrapper catches VmSuspended, builds
  suspension object, and calls _driveAsync directly

Toggle and count clicks work fully. Bounce adds class but wait/remove
requires IO suspension in CEK context (eval-expr-cek doesn't support
perform — needs VM-path evaluation in hs-handler).

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

hosts/ocaml/browser/sx_browser.ml

@@ -108,6 +108,23 @@ let rec value_to_js (v : value) : Js.Unsafe.any =
         let result = call_sx_fn v args in
         value_to_js result
       with
+      | Sx_vm.VmSuspended (request, vm) ->
+        (* Build {suspended, request, resume} and hand to _driveAsync *)
+        let obj = Js.Unsafe.obj [||] in
+        Js.Unsafe.set obj (Js.string "suspended") (Js.Unsafe.inject Js._true);
+        Js.Unsafe.set obj (Js.string "request") (value_to_js request);
+        Js.Unsafe.set obj (Js.string "resume") (Js.wrap_callback (fun result_js ->
+          let result = js_to_value result_js in
+          try value_to_js (Sx_vm.resume_vm vm result)
+          with Eval_error msg ->
+            ignore (Js.Unsafe.meth_call
+              (Js.Unsafe.get Js.Unsafe.global (Js.string "console"))
+              "error" [| Js.Unsafe.inject (Js.string ("[sx] resume: " ^ msg)) |]);
+            Js.Unsafe.inject Js.null));
+        let drive = Js.Unsafe.get Js.Unsafe.global (Js.string "_driveAsync") in
+        if not (Js.Unsafe.equals drive Js.undefined) then
+          ignore (Js.Unsafe.fun_call drive [| Js.Unsafe.inject obj |]);
+        Js.Unsafe.inject obj
       | Eval_error msg ->
         let fn_info = Printf.sprintf " [callback %s handle=%d]" (type_of v) handle in
         ignore (Js.Unsafe.meth_call