codex: refactor pyi parser to ast visitor

[saidctb]

Summary

Test plan

• PYTHONPATH=. pytest -q

Merge checklist

• All required checks are green (tests + parser-reference-guard).
• If parser Fortran data or goldens changed, the PR explains why.
• If using ignore-parser-reference-guard, the PR explains why it is safe.

[codecov]

Codecov Report

❌ Patch coverage is 97.47292% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
semantics/pyi_parser.py	97.47%	4 Missing and 3 partials ⚠️

Flag	Coverage Δ
py312	96.74% <97.47%> (+0.37%)	⬆️

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Files with missing lines	Coverage Δ
semantics/pyi_parser.py	97.04% <97.47%> (+2.64%)	⬆️

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