Add posterior diagnostics and ECM vectorization tools

[zhechengyin]

Summary

• Add posterior quality diagnostics for Bayesian inference results.
• Add ECM topology simplification and vectorization utilities.
• Add documentation and tests for the new diagnostics and ECM APIs.

Validation

• Ran targeted smoke tests for diagnostics and ECM utilities.
• Ran py_compile on the new modules and tests.
• Ran ruff check on the new diagnostics/ECM files and tests.

Note: Full pytest coverage depends on the local environment using the editable checkout rather than the preinstalled autoeis package.