New 166-line test file for variadic MixedDOF type system:
- Tests type-level multi-field composition with @MixedDOF macro
- Validates DOF indices structure (tuple of tuples, not NamedTuple)
- Tests DOF sharing between elements for different entity types
- Compares MixedDOF (vararg) vs NamedTuple approaches
- Demonstrates 4-field mixed system: Temperature, Displacement,
Pressure, Electric potential on different entities
Validates variadic approach to multi-field elements where DOF types
are mixed as type parameters rather than named fields.
Jukka Aho