Prepare Solver Infrastructure (Enumerations and Tags)

[lsawade]

• Create MEDIUM_TAG_ELASTIC_SPIN macro in core/specfem/macros/material_iterators.hpp (id 7 or next available), mapping to specfem::element::medium_tag::elastic_spin
• Update MEDIUM_TAGS_DIM3 macro to add (DIMENSION_TAG_DIM3, MEDIUM_TAG_ELASTIC_SPIN)
• Update MATERIAL_SYSTEMS_DIM3 macro to add (DIM3, ELASTIC_SPIN, ISOTROPIC_COSSERAT, NONE)
• Update ELEMENT_TYPES_DIM3 macro to add (DIM3, ELASTIC_SPIN, ISOTROPIC_COSSERAT, NONE)
• Add template specialization attributes<dim3, elastic_spin> in core/specfem/element/attributes.hpp with components = 6, has_cosserat_stress = true, has_cosserat_couple_stress = true, has_damping_force = false
• Add elastic_spin to the is_elastic type trait in core/specfem/element/attributes.hpp
• Verify elastic_spin string conversion completeness in core/specfem/element/to_string.cpp — the to_string() switch currently falls to "unknown" for elastic_spin, needs an explicit case; from_string() already handles it at line 146
• Add isotropic_cosserat case to to_string(property_tag) in core/specfem/element/to_string.cpp — currently falls to "unknown"