fixed polynomial fit derivative error

[markleader]

Summary

Correct analytic equilibrium derivatives that caused large relative errors in species sensitivities.

Changes

• Fix the NASA polynomial enthalpy derivative’s (a_1/T^2) coefficient.
• Correct reduced equilibrium Jacobian and explicit-input terms.
• Add regression tests for thermodynamic derivative identities.

Testing

Full fd_convergence.py sweep: worst large-species error (2.72\times10^{-5}).
pytest source/bind/python/tests: 68 passed.
ctest --test-dir build-dev: 15 passed.

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes: analytic derivative results are corrected; forward equilibrium solutions are unchanged.