perf(GraphEngine): using caching clawed back ~10% performance

src/include/gridfire/engine/engine_graph.h

@@ -875,6 +875,7 @@ namespace gridfire::engine {
         mutable CppAD::ADFun<double> m_rhsADFun; ///< CppAD function for the right-hand side of the ODE.
         mutable CppAD::ADFun<double> m_epsADFun; ///< CppAD function for the energy generation rate.
         mutable CppAD::sparse_jac_work m_jac_work; ///< Work object for sparse Jacobian calculations.
+        mutable std::vector<double> m_local_abundance_cache;
 
         bool m_has_been_primed = false; ///< Flag indicating if the engine has been primed.
 

src/include/gridfire/reaction/reaction.h

@@ -809,6 +809,8 @@ namespace gridfire::reaction {
         std::vector<RateCoefficientSet> m_rates; ///< List of rate coefficient sets from each source.
         bool m_weak = false;
 
+        mutable std::unordered_map<double, double> m_cached_rates;
+
     private:
         /**
          * @brief Template implementation for calculating the total reaction rate.