Fix VM reuse_stack lost across stub VM boundary on IO suspension

Root cause: when perform fires inside a VM closure chain (call_closure_reuse),
the caller frames are saved to reuse_stack on the ACTIVE VM. But the
_cek_io_suspend_hook and _cek_eval_lambda_ref create a NEW stub VM for the
VmSuspended exception. On resume, resume_vm runs on the STUB VM which has
an empty reuse_stack — the caller frames are orphaned on the original VM.

Fix: transfer reuse_stack from _active_vm to the stub VM before raising
VmSuspended. This ensures resume_vm -> restore_reuse can find and restore
the caller's frames after async resume via _driveAsync/setTimeout.

Also restore step_limit/step_count refs dropped by bootstrap.py regeneration.

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

hosts/ocaml/lib/sx_ref.ml

@@ -10,7 +10,9 @@ open Sx_runtime
 let trampoline_fn : (value -> value) ref = ref (fun v -> v)
 let trampoline v = !trampoline_fn v
 
-
+(* Step limit for timeout protection *)
+let step_limit : int ref = ref 0
+let step_count : int ref = ref 0
 
 (* === Mutable globals — backing refs for transpiler's !_ref / _ref := === *)
 let _strict_ref = ref (Bool false)