Island body: letrec instead of define (fixes render-to-dom), host-object JS fns

runner.sx: Converted define forms inside island body to letrec. Multiple
define forms in a let body cause render-to-dom to fall back to eval-expr
for the whole body, which evaluates (div ...) as a list instead of
rendering it to DOM. letrec keeps the last body expression (div) as the
render target.

sx_browser.ml: js_to_value now stores plain JS functions as host objects
(Dict with __host_handle) instead of wrapping as NativeFn. This preserves
the original JS function identity through the SX→JS round-trip, keeping
_driveAsync wrappers from host-callback intact when passed to
addEventListener via host-call.

Remaining: IO suspension in click handler is caught as "IO suspension in
non-IO context" instead of being driven by _driveAsync. The host-callback
wrapper creates the right JS function, but the event dispatch path doesn't
go through K.callFn.

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

hosts/ocaml/browser/sx_browser.ml

@@ -171,10 +171,14 @@ and js_to_value (js : Js.Unsafe.any) : value =
       if not (Js.Unsafe.equals h Js.undefined) then
         get_handle (Js.float_of_number (Js.Unsafe.coerce h) |> int_of_float)
       else
-        (* Plain JS function — wrap as NativeFn *)
-        NativeFn ("js-callback", fun args ->
-          let js_args = args |> List.map value_to_js |> Array.of_list in
-          js_to_value (Js.Unsafe.fun_call js (Array.map Fun.id js_args)))
+        (* Plain JS function — store as host object so value_to_js
+           returns the ORIGINAL JS function when passed to host-call.
+           This preserves wrappers like _driveAsync that host-callback
+           attaches for IO suspension handling. *)
+        let id = host_put js in
+        let d = Hashtbl.create 2 in
+        Hashtbl.replace d "__host_handle" (Number (float_of_int id));
+        Dict d
     | "object" ->
       let h = Js.Unsafe.get js (Js.string "__sx_handle") in
       if not (Js.Unsafe.equals h Js.undefined) then