Fix bytecode when/do/perform: snapshot pending_cek in resume closure

Root cause: nested cek_call_or_suspend calls on the same VM (from
synchronous callbacks like dom-listen firing handler immediately)
overwrote pending_cek before the first resume ran.

Fix: _vm_suspension_to_dict snapshots pending_cek at capture time
and restores it in the resume closure before calling resume_vm.
This ensures each suspension's CEK state is preserved regardless
of nested overwrite.

test_bytecode_repeat.js: 4/4 pass (was 3/4).
Source: 6 suspensions ✓  Bytecode: 6 suspensions ✓

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

hosts/ocaml/lib/sx_vm.ml

@@ -814,12 +814,10 @@ and run vm =
 let resume_vm vm result =
   (match vm.pending_cek with
    | Some cek_state ->
-     (* Resume the suspended CEK evaluation first *)
      vm.pending_cek <- None;
      let final = Sx_ref.cek_resume cek_state result in
      (match Sx_runtime.get_val final (String "phase") with
       | String "io-suspended" ->
-        (* CEK suspended again — re-suspend the VM *)
         vm.pending_cek <- Some final;
         raise (VmSuspended (Sx_runtime.get_val final (String "request"), vm))
       | _ ->
@@ -948,12 +946,18 @@ let () = _vm_call_closure_ref := (fun cl args -> call_closure_reuse cl args)
 let () = _vm_suspension_to_dict := (fun exn ->
   match exn with
   | VmSuspended (request, vm) ->
+    (* Snapshot pending_cek NOW — a nested cek_call_or_suspend on the same VM
+       may overwrite it before our resume function is called. *)
+    let saved_cek = vm.pending_cek in
     let d = Hashtbl.create 3 in
     Hashtbl.replace d "__vm_suspended" (Bool true);
     Hashtbl.replace d "request" request;
     Hashtbl.replace d "resume" (NativeFn ("vm-resume", fun args ->
       match args with
       | [result] ->
+        (* Restore the saved pending_cek before resuming — it may have been
+           overwritten by a nested suspension on the same VM. *)
+        vm.pending_cek <- saved_cek;
         (try resume_vm vm result
          with exn2 ->
            match !_vm_suspension_to_dict exn2 with