Async error handler: dispatch Eval_error to VM handler_stack in resume_vm

When an error occurs during resumed VM execution (after perform/hs-wait),
resume_vm now checks the VM's handler_stack. If a handler exists (from a
compiled guard form's OP_PUSH_HANDLER), it unwinds frames and jumps to
the catch block — exactly like OP_RAISE. This enables try/catch across
async perform/resume boundaries.

The guard form compiles to OP_PUSH_HANDLER which lives on the vm struct
and survives across setTimeout-based async resume. Previously, errors
during resume escaped to the JS console as unhandled exceptions.

Also restored guard in the test runner (was cek-try which doesn't survive
async) and restored error-throwing assertions in run-action.

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

hosts/ocaml/lib/sx_vm.ml

@@ -852,10 +852,26 @@ let resume_vm vm result =
    | None ->
      push vm result);
   (try run vm
-   with VmSuspended _ as e ->
-     (* Re-suspension during resume: the VM hit another perform.
-        The new VmSuspended carries the current VM state. *)
-     raise e);
+   with
+   | VmSuspended _ as e ->
+     (* Re-suspension during resume: the VM hit another perform. *)
+     raise e
+   | Eval_error msg ->
+     (* Error during resumed execution. If the VM has a handler on its
+        handler_stack, dispatch to it (same as OP_RAISE). This enables
+        try/catch across async perform/resume boundaries — the handler
+        was pushed before the perform and survives on the vm struct. *)
+     (match vm.handler_stack with
+      | entry :: rest ->
+        vm.handler_stack <- rest;
+        while List.length vm.frames > entry.h_frame_depth do
+          match vm.frames with _ :: fs -> vm.frames <- fs | [] -> ()
+        done;
+        vm.sp <- entry.h_sp;
+        entry.h_frame.ip <- entry.h_catch_ip;
+        push vm (String msg);
+        run vm
+      | [] -> raise (Eval_error msg)));
   (* Clear reuse_stack — any entries here are stale from the original
      suspension and don't apply to the current state. The VM just
      completed its execution successfully. *)