perf(precomputation): cleaned up allocations

recovered about 5% execution time

src/include/gridfire/engine/engine_abstract.h

@@ -53,7 +53,7 @@ namespace gridfire::engine {
     struct StepDerivatives {
         std::map<fourdst::atomic::Species, T> dydt{}; ///< Derivatives of abundances (dY/dt for each species).
         T nuclearEnergyGenerationRate = T(0.0); ///< Specific energy generation rate (e.g., erg/g/s).
-        std::map<fourdst::atomic::Species, std::unordered_map<std::string, T>> reactionContributions{};
+        std::optional<std::map<fourdst::atomic::Species, std::unordered_map<std::string, T>>> reactionContributions = std::nullopt;
         T neutrinoEnergyLossRate = T(0.0); // (erg/g/s)
         T totalNeutrinoFlux = T(0.0);      // (neutrinos/g/s)