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/browser/sx_browser.ml

@@ -132,11 +132,15 @@ let rec value_to_js (v : value) : Js.Unsafe.any =
               (* Return suspension object — the JS driveAsync caller handles scheduling *)
               Js.Unsafe.inject (make_suspension req2 vm2)
             | Eval_error msg ->
-              (* Enhanced error: show pending_cek kont and reuse_stack info *)
+              (* Enhanced error: show pending_cek kont, reuse_stack, and VM frame info *)
-              let extra = Printf.sprintf " [vm: pending_cek=%b reuse=%d frames=%d]"
+              let vm_frame_names = String.concat "," (List.map (fun f ->
+                match f.Sx_vm.closure.Sx_types.vm_name with Some n -> n | None -> "?"
+              ) v.Sx_vm.frames) in
+              let extra = Printf.sprintf " [vm: pending_cek=%b reuse=%d frames=[%s] sp=%d]"
                 (v.Sx_vm.pending_cek <> None)
                 (List.length v.Sx_vm.reuse_stack)
-                (List.length v.Sx_vm.frames) in
+                vm_frame_names
+                v.Sx_vm.sp in
               ignore (Js.Unsafe.meth_call
                 (Js.Unsafe.get Js.Unsafe.global (Js.string "console"))
                 "error" [| Js.Unsafe.inject (Js.string ("[sx] resume: " ^ msg ^ extra)) |]);
@@ -549,9 +553,17 @@ let rec make_js_callFn_suspension request vm =
     | Sx_vm.VmSuspended (req2, vm2) ->
       Js.Unsafe.inject (make_js_callFn_suspension req2 vm2)
     | Eval_error msg ->
+      let vm_frame_names = String.concat "," (List.map (fun f ->
+        match f.Sx_vm.closure.Sx_types.vm_name with Some n -> n | None -> "?"
+      ) vm.Sx_vm.frames) in
+      let extra = Printf.sprintf " [vm: pending_cek=%b reuse=%d frames=[%s] sp=%d]"
+        (vm.Sx_vm.pending_cek <> None)
+        (List.length vm.Sx_vm.reuse_stack)
+        vm_frame_names
+        vm.Sx_vm.sp in
       ignore (Js.Unsafe.meth_call
         (Js.Unsafe.get Js.Unsafe.global (Js.string "console"))
-        "error" [| Js.Unsafe.inject (Js.string ("[sx] resume: " ^ msg)) |]);
+        "error" [| Js.Unsafe.inject (Js.string ("[sx] resume: " ^ msg ^ extra)) |]);
       Js.Unsafe.inject Js.null));
   obj
 

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. *)

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-7899047a",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-21fac337",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-7f504b3c",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-b285d4f3",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new