Add a Hellan-Herrmann-Johnson basis

Summary

This merge request adds a HellanHerrmannJohnsonBasis for simplex grids, including the BasisFactory::hhj() and BasisFactory::hellanHerrmannJohnson() factories.

The supported scope is deliberately limited to polynomial orders zero through six in dimensions one and two:

• In 2D this is the classical symmetric matrix-valued HHJ element on triangles with continuous normal-normal traces.
• In 1D a symmetric 1-by-1 matrix is a scalar. The implementation uses the scalar Lagrange basis with the double contravariant Piola transformation. This degenerate analogue is useful, for example, in shape-operator discretizations on embedded curves, but is not presented as a standard Symfem HHJ element.

Three-dimensional HHJ elements are mathematically well-defined and are implemented on tetrahedra by Symfem and FIAT. They are outside this merge request because the Dune implementation still needs generated tetrahedral shape functions and globally consistent face-DOF permutations.

Implementation details

• Add reference basis evaluation, double-divergence evaluation, local interpolation, local coefficients, and global DOF assembly.
• Apply the double contravariant Piola transformation, including the scalar 1D case and embedded-grid geometries.
• Orient the edge moments consistently across neighboring triangles.
• Add the expected evaluateJacobian interface. It currently reports Dune::NotImplemented, because differentiating the geometry-dependent transformation is not part of this implementation.
• Generate the 1D and 2D reference evaluations with doc/CodeGeneration_HHJ.py using the Symfem dune Lagrange variant.
• Construct generated SPDX annotations at runtime so reuse lint does not interpret them as annotations belonging to the generator itself.
• Install the generated hellanherrmannjohnsonbasis_inc.hh header.
• Test direct and basis-factory construction, global normal-normal continuity, local interpolation duality, and reproduction of constants.
• Add a changelog entry documenting the supported scope.

Follow-up work

• Implement transformed first derivatives instead of throwing from evaluateJacobian.
• Add the tetrahedral reference basis and global face orientation/permutation logic before enabling 3D.