codex: add C fixture scaffolding and raw metadata coverage

[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 93.33333% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
c_parser/parser.py	92.30%	0 Missing and 3 partials ⚠️

Flag	Coverage Δ
py312	92.62% <93.33%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
c_parser/models.py	98.75% <ø> (ø)
c_parser/preprocessor.py	92.30% <100.00%> (+0.78%)	⬆️
c_parser/parser.py	90.83% <92.30%> (ø)

... and 1 file with indirect coverage changes

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