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

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

libmeanfield/interface/operators/kernels/gravity_kernels.cppm

@@ -5,6 +5,13 @@ export import :mapping.domain_mapper;
 export import :fem;
 
 export namespace mean_field::operators::kernels {
+    /*
+     * Kernel boundary contract
+     * ------------------------
+     * These low-level element kernels operate on MFEM true-DOF vectors. The
+     * prepared operators and gravity contexts own FieldDof maps and are the
+     * only layer that gathers/scatters reduced public coordinates.
+     */
 
     void apply_mapped_hdiv_mass(
         const fem::FEM &f,