feat(preconditioner): major work on preconditioner system

first preconditioner MVP

libmeanfield/interface/operators/prepared_barotropic_closure_operator.cppm

@@ -54,6 +54,10 @@ export namespace mean_field::operators {
 
         void BuildResidual(mfem::Vector &residual) const;
 
+        // Exact diagonal of the prepared density-to-closure block, expressed
+        // in the reduced density coordinates used by the root operator.
+        void AssembleDensityJacobianDiagonal(mfem::Vector &diagonal) const;
+
         [[nodiscard]] bool IsPrepared() const noexcept;
         [[nodiscard]] std::uint64_t GetPreparationCount() const noexcept;
         [[nodiscard]] int GetDensitySize() const noexcept;