assignment to identifiers bound by for-like forms can break invariants

Identifiers bound by for- and for/fold-like forms—for both accumulators and sequence elements—can be set!. Assigning to these variables can break invariants or expose implementation details of a for-like form or for-clause transformer (as in define-sequence-syntax).

Some examples:

1. #lang racket
   
   (for/lists (acc)
              ([v (in-range 2)])
      (unless (zero? v)
        (set! acc 'bad))
      v)
   

   ->

   racket/private/for.rkt:1963:23: reverse: contract violation
     expected: list?
     given: '(1 . bad)
   

2. #lang racket
   
   (require (for-syntax syntax/parse))
   
   (define-sequence-syntax in-my-naturals
     (λ () #'in-naturals)
     (syntax-parser
       [[(n) (_)]
        #'[(n) (:do-in ([(n) 0])
                       #t
                       ([n n])
                       #t
                       ([(n) n])
                       #t
                       #t
                       ((add1 n)))]]))
   
   (for/list ([i (in-my-naturals)]
              #:when (< i 3))
     (set! i 'bad))
   

   ->

   add1: contract violation
     expected: number?
     given: 'bad
   

Start by reproducing both examples and inspect racket/private/for.rkt around the reported line, along with the for-like and for-clause transformer behavior shown in the issue. Determine the intended handling of set! on accumulator and sequence-element bindings, then add coverage demonstrating that these assignments cannot break the form's invariants or expose implementation details.