Add PySCF reference comparison tooling

[mveril]

Summary

• add an optional PySCF comparison script for RustiQ sample energies
• document all-case and single-case usage with standard Python
• keep the reference checks separate from the Rust test suite

Validation

• Python 3.14 syntax compilation passed
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features

The full PySCF comparisons require the Nix scientific Python environment and will be validated on Linux.