perf(engine_multiscale): performance enhancments due to improved hashing, locality, and data structure optimization

This particular commit speeds up QSE solving for systems where reverse reactions and engine caching is disabled by about 24%

src/include/gridfire/reaction/weak/weak_interpolator.h

@@ -116,12 +116,6 @@ namespace gridfire::rates::weak {
         ) const;
     private:
         quill::Logger* m_logger = fourdst::logging::LogManager::getInstance().getLogger("log");
-        /**
-         * @brief Pack (A,Z) into a 32-bit key used for the internal map.
-         *
-         * Layout: (A << 8) | Z. To unpack, use (key >> 8) for A and (key & 0xFF) for Z.
-         */
-        static uint32_t pack_isotope_id(uint16_t A, uint8_t Z);
 
         /**
          * @brief Per-isotope grids over (T9, log10(rho*Ye), mu_e) with payloads at lattice nodes.