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Single assignment restriction of Python 1 and Python 2: specs and corrected implementation (#208)

* single assignment

* Fix python_1.tex: drop unrelated typing-table/python_math changes that leaked in

These lines (the python_typing_front/middle_12/back split and the
python_math input) belong to the in-progress operators branch, not to
single assignment, and don't exist as files on this branch — restore
python_1.tex to match main aside from the single-assignment input line.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* gemini review

* Enforce the single-assignment restriction on function/lambda parameters

The spec (docs/specs/python_single_assignment.tex) says a parameter must
not appear on the left-hand side of an assignment within its scope, but
createNoReassignmentValidator only tracked names first seen via an
Assign/AnnAssign/FunctionDef node, so a parameter reassigned by a plain
`=` in the body (e.g. `def f(x): x = 1`) went undetected at Python §1/§2 —
in both the CSE machine and the stepper, since both delegate name
resolution and chapter feature-gating to the same analyze()/Resolver
pipeline.

Environment gains a `parameters` set (the function/lambda's own
parameter names, distinct from `names`, which also accumulates names
assigned in the body). The reassignment validator now seeds each scope's
"already declared" set from it, so assigning a parameter is flagged
exactly like re-assigning any other declared name — while chapters 3/4,
where reassignment is allowed, are unaffected since the validator isn't
wired in there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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