Add Python RHF, UHF, and MP2 bindings

[mveril]

Summary

• add Python Atom, Geometry, and Molecule bindings with tuple-based coordinates and geometry transformations
• add typed BasisSet, RHF, UHF, HfResult, and Mp2Result APIs backed by the existing calculation engine
• support explicit Bohr/Angstrom units, convergence options, MP2 frozen orbitals, and VS Code type stubs
• add Python documentation and pytest coverage, including PySCF reference comparisons

Validation

• cargo test
• cargo check -p rustiq-python
• cargo clippy -p rustiq-python --all-targets -- -D warnings
• Python pytest suite: 19 passed

No changes remain in rustiq-core; the binding uses its existing public calculation API.