Add native spherical harmonic surface coordinates to coupled equilibrium

libmeanfield/interface/deformation/surface_metric.cppm

@@ -0,0 +1,18 @@
+module;
+
+#include <mfem/linalg/vector.hpp>
+
+export module mean_field:deformation.surface_metric;
+
+export import :deformation.domain_deformation;
+export import :fem;
+
+export namespace mean_field::deformation {
+    // A reference parameter metric depends on the surface tangent, not the
+    // interior mesh lift. Assembly is out of line to keep MFEM quadrature
+    // implementation headers out of every normalization/preconditioner user.
+    [[nodiscard]] mfem::Vector assembleSurfaceParameterMassDiagonal(
+        const fem::FEM &finiteElements,
+        const PreparedDomainDeformationRuntime &deformation
+    );
+} // namespace mean_field::deformation