[codex] add C parser skeleton CLI - #13

Summary

Adds the initial C parser skeleton behind explicit C-mode entrypoints and CLI flags.

Changes

• Add the c_parser package with typed skeleton models, CParseError, CParser, parse_c_file, and parse_c_project.
• Wire x2py --language c --parse to stable skeleton human and JSON output while keeping Fortran as the default.
• Reject unsupported C semantic, .pyi, and wrap-readiness stages until later phases.
• Add focused C skeleton CLI/API tests while leaving broader C roadmap tests skipped.
• Update docs/c_parser/ to reflect skeleton status and require future C parser work to update all affected docs in the same change.

Impact

Users can now discover and run explicit C parse skeleton commands without changing existing Fortran behavior. The skeleton does not claim real C declaration parsing yet; all parser sections remain empty with parser_status: skeleton.

Validation

• python3 -m pytest -q tests/parser/c/test_c_cli_skeleton.py tests/parser/c/test_c_public_api_skeleton.py tests/parser/test_cli.py
• python3 -m pytest -q

Full-suite result from the final code: 2985 passed, 84 skipped.