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Design note: the environment stepper (e-stepper) (#507)

* Design note: the environment stepper (e-stepper)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* e-stepper note: dynamic globals, free-variable errors, is not / in

Address the Codex review: the global frame grows as assignments execute
(builtin shadowing), an unassigned binding in an enclosing frame is a
free-variable NameError rather than UnboundLocalError, and add rules for
is not, in and not in, which the substitution stepper rejects.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

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