XpongeCPP v0.2.8
Compatibility target: Xponge 1.7.1.
This patch restores Python instance attributes on the pybind-backed Assign type, allowing RESP fitting to retain charge_fit_diagnostics and charge_fit_metadata. It fixes SPONGE input generation failures seen after RESP parameterization and adds regression coverage for the compatibility contract.
Validation includes the complete compatibility suite against official Xponge 1.7.1, a real PySCF RESP calculation, cross-platform CI, source distribution checks, and isolated wheel installation.