Diagnostic: enhanced resume error with VM frame names, clear stale reuse on re-suspend

The Not callable: nil error happens on a stub VM (frames=[], sp=0) during
cek_resume with 12 CEK kont frames. The error is from a reactive signal
subscriber (reset! current ...) that triggers during run vm after resume.
The subscriber callback goes through CEK via cek_call_or_suspend and the
CEK continuation tries to call nil.

This is a reactive subscriber notification issue, not a perform/resume
frame management issue. The VM frames are correctly restored — the error
happens during a synchronous reset! call within the resumed VM execution.

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

hosts/ocaml/lib/sx_vm.ml

@@ -270,6 +270,7 @@ let jit_compile_comp ~name ~params ~has_children ~body ~closure globals =
     Saves the suspended CEK state in vm.pending_cek for later resume. *)
 let cek_call_or_suspend vm f args =
   incr _vm_cek_count;
+  (* Removed debug trace *)
   let a = match args with Nil -> [] | List l -> l | _ -> [args] in
   (* Replace _active_vm with an empty isolation VM so call_closure_reuse
      inside the CEK pushes onto an empty frame stack rather than the caller's.
@@ -850,7 +851,14 @@ let resume_vm vm result =
         push vm (Sx_ref.cek_value final))
    | None ->
      push vm result);
-  run vm;
+  (try run vm
+   with VmSuspended _ as e ->
+     (* Re-suspension during resume: the VM hit another perform.
+        Clear reuse_stack — these entries are stale from the PREVIOUS
+        suspension and don't apply to the current VM frame state.
+        The new VmSuspended carries the current VM state correctly. *)
+     vm.reuse_stack <- [];
+     raise e);
   (* Restore call_closure_reuse continuations saved during suspension.
      reuse_stack is in catch order (outermost first from prepend) —
      reverse to get innermost first, matching callback→caller unwinding. *)