Step 5.5 phase 4 (partial): add OP_PERFORM to vm.sx spec

Adds opcode 112 (OP_PERFORM / IO suspension) to lib/vm.sx's vm-step
dispatch. The native sx_vm.ml already has this opcode — this brings
the SX spec into alignment.

Full VM transpilation deferred to Step 6 (define-record-type): the
dict-based state in vm.sx maps to get_val/sx_dict_set_b calls which
are ~5x slower than native record/array access in the hot loop.
define-record-type will let vm.sx use typed records that the
transpiler maps to OCaml records natively.

2598/2598 tests passing.

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

lib/vm.sx

@@ -516,6 +516,10 @@
         (vm-push vm (inc (vm-pop vm)))
         (= op 175)
         (vm-push vm (dec (vm-pop vm)))
+        (= op 112)
+        (let
+          ((request (vm-pop vm)))
+          (error (str "VM: IO suspension (OP_PERFORM) — request: " request)))
         :else (error (str "VM: unknown opcode " op))))))
 
 (define