feat(preconditioner): major work on preconditioner system

first preconditioner MVP

libmeanfield/interface/operators/prepared_hydrostatic_equilibrium_operator.cppm

@@ -138,6 +138,10 @@ export namespace mean_field::operators {
             mfem::Vector &action
         ) const;
 
+        // Exact diagonal of the volume enthalpy-to-hydrostatic block. Surface
+        // boundary-row replacement is deliberately applied by its owner.
+        void AssembleEnthalpyJacobianDiagonal(mfem::Vector &diagonal) const;
+
         [[nodiscard]] bool IsPrepared() const noexcept;
 
         [[nodiscard]] const context::hydrostatic::HydrostaticPreparationStatistics &