Add real-world cookbook examples

[cmccomb]

Summary

• add breast cancer and diabetes cookbook examples backed by the bundled CSV datasets
• document and expose the new examples through the cookbook module and README
• provide reusable dataset loaders plus regression tests to ensure the CSV fixtures parse as expected

Testing

• cargo fmt
• cargo clippy --all-targets -- -D warnings -D clippy::pedantic
• cargo test

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Codex Task