v0.3.1

Bug fix

• vqe() now case-insensitive for Pauli ops strings — lowercase ops like 'zz' or 'zi' silently returned wrong values in v0.3.0. The all-identity fast-path check (/[XYZ]/) and all Pauli comparisons were uppercase-only, while expectation() already normalised to uppercase. Now consistent: vqe() accepts any case.