docs: refresh security and C standards (#6)

* docs: refresh security and C standards

Update the security analysis to separate current Python-only behavior from planned native, REST, and agent surfaces. Refresh the C coding standards with a status note, review checklist, fuzzing guidance, and ABI compatibility expectations. Add lightweight cross-links from README and CONTRIBUTING.\n\nCo-Authored-By: Claude <noreply@anthropic.com>

* docs: fix stale section reference and add trailing newline

Address review feedback on PR #6:

- Update the /std:c17 rationale cross-reference from Section 9 to Section 10

  after the Review Checklist insertion renumbered the C coding standards doc.

- Restore the trailing newline at the end of SECURITY.md.

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Codex Reviewer <lcj.codex.review@proton.me>
Co-authored-by: Agent for Leonard Janke <leonard.janke.agent@proton.me>
Co-authored-by: Claude <noreply@anthropic.com>