Enhance autodiff robustness and unique semantics

[briangu]

Summary

• introduce an AutodiffNotSupported exception and expand the Dual number implementation with richer ufunc coverage and Python operator support
• narrow the autodiff fallback logic and exercise it with new unit tests covering additional ufuncs, pure Python expressions, and numeric fallbacks
• replace the string-based UNIQUE implementation with structural comparisons and add regression tests to ensure mixed-type elements are preserved

Testing

• python3 -m unittest

---

https://chatgpt.com/codex/tasks/task_e_68c6bf0978bc83328aa773b875335b6e