[codex] Add C parser planning roadmap

[saidctb]

Summary

Adds the initial C parser planning documentation under docs/c_parser/ on the isolated c-parser/main integration branch.

This includes:

• C parser architecture plan based on the existing Fortran parser architecture
• CLI workflow plan with explicit early --language c --parse mode
• future C parser reference document
• detailed phase-by-phase implementation checklist
• clarified parser-first model policy for C-specific facts in c_parser/models.py
• clarified preprocessing policy: raw-source parsing first, compiler-assisted preprocessing for macro-heavy APIs later
• clarified callback/function-pointer readiness policy requiring user-supplied .pyi callback policy before wrapping

Branch Strategy

This PR targets main only to make the planning branch visible on GitHub. The C parser implementation itself should remain isolated on c-parser/main until mature and stable. Future work should branch from and merge back into c-parser/main, not directly into project main.

Validation

• git diff --check passed
• Docs-only change; no parser logic, CLI logic, fixtures, or generated goldens were added
• Existing Fortran parser behavior is untouched

[saidctb]

Closed because C parser work must not target project main. The pushed planning branch remains available as c-parser/main, and future C parser PRs should target c-parser/main only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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