Update citation to SSRN preprint

[Str4vinci]

Summary

• update the README citation to the SSRN preprint
• add the SSRN DOI and abstract URL to CITATION.cff

Checks

• uv run python -c "import pathlib, yaml; yaml.safe_load(pathlib.Path('CITATION.cff').read_text())"
• git diff --check -- README.md CITATION.cff

[Str4vinci]

Closing because this citation update was opened from an unrelated Codex branch. Replaced by #53 from docs/preprint-citation.