Release XpongeCPP 0.2.8

[xiaoxuan-yu]

Summary

• allow pybind-backed Assign instances to retain Python compatibility attributes
• restore RESP charge_fit_diagnostics and charge_fit_metadata persistence
• align release metadata and compatibility target with Xponge 1.7.1

Validation

• full suite against official Xponge 1.7.1: 704 passed, 29 skipped, 1 xfailed
• Assign compatibility cohort: 59 passed, 8 skipped
• release-focused tests: 22 passed
• real PySCF RESP calculation: charge sum 0, max constraint residual 0
• wheel/sdist build and twine check passed
• isolated wheel install confirmed version and dynamic attributes