Fix memory safety and code structure issues in self-rewriting Lisp program

[Copilot]

Addresses code review feedback on the robust self-rewriting Lisp implementation, fixing mutation bugs, nil safety, and code ordering issues.

Changes

• Eliminate shared structure mutation in increment-generation-in-sexprs: Create modified copy via mapcar instead of mutating input list with setf
• Add nil safety to verify-self-modification: Check gen-form existence before dereferencing to prevent type errors
• Fix code insertion order in transform-code-additions: Replace push (prepends) with append to add new definitions at end of file, preserving shebang and structure
• Optimize lookup efficiency: Use let* binding to eliminate duplicate find-defparameter-form call

;; Before: mutates shared structure
(setf (third gen-form) (1+ (third gen-form)))

;; After: returns modified copy
(mapcar (lambda (form)
          (if (and (listp form) (eq (car form) 'defparameter) ...)
              `(defparameter *generation* ,(1+ (third form)))
              form))
        sexprs)

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