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Stepper: reject chapters above 2 during preprocessing (#486)

The substitution stepper only exists for Python §1 and §2 (no stepper
evaluator is built or exported for a higher chapter), but preprocessPython
accepted any chapter and validated a §3/§4 program (reassignment, loops,
lists, rest parameters, ...) as though it could be stepped.

Return a clear error from preprocessPython, the one entry point every caller
goes through, for any chapter above MAX_STEPPER_CHAPTER (2), before any other
check.

The range-reservation test no longer checks the stepper at chapters 3 and 4:
that is not a valid stepper configuration.

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

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