Defer block cleanup

defer statement schedules work when the enclosing block exits. Defers are block-scoped and run in reverse registration order on fall-through, return, propagation, break, or continue.

They read current values at exit time. Runtime errors abort without running them, and a defer body cannot redirect control with return, propagation, or await.

fn main() {
  defer println("first")
  defer println("second")
  println("work")
}