feat(support): continued migration to new field dof support system

mass normaliztion, gravity, displacement, and hydrostatic equilibrium are now migrated

libmeanfield/interface/operators/prepared_mass_normalization.cppm

@@ -61,6 +61,8 @@ export namespace mean_field::operators {
      * Density and displacement are borrowed from the shared gravity-field
      * linearization context. This keeps the mass row on exactly the same
      * frozen state and geometry as the gravity and mechanical rows.
+     * Jacobian directions use the corresponding registered FieldDof
+     * coordinates; expansion to MFEM true-DOF vectors is private.
      */
     class PreparedMassNormalizationOperator final {
     public:
@@ -153,6 +155,9 @@ export namespace mean_field::operators {
         MassNormalizationDependencies m_preparedDependencies;
         mfem::Vector m_cachedResidual;
 
+        mutable mfem::Vector m_densityVariationTrue;
+        mutable mfem::Vector m_displacementVariationTrue;
+
         double m_currentMass{0.0};
         double m_targetMass{0.0};