New file: test/domains/continuum/test_dofs_per_node.jl
Tests for:
- dofs_per_node(field) for various field types
- Displacement{3} → 3 DOFs per node
- Temperature → 1 DOF per node
- DisplacementRotation → 6 DOFs per node (future)
Validates correct DOF count for field types used in
DOF mapping and cache dimensioning.