codex: add C function definition start and end locations

[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

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
py312	92.62% <100.00%> (ø)

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Files with missing lines	Coverage Δ
c_parser/lexer.py	100.00% <ø> (ø)
c_parser/models.py	98.75% <ø> (ø)
c_parser/parser.py	90.83% <100.00%> (ø)

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