New file: test/domains/continuum/test_dof_mapping.jl
Tests for:
- get_dof_mapping!(dofs, node_ids, field)
- Maps node IDs to global DOF indices
- Validates displacement field (3 DOF/node)
- Validates temperature field (1 DOF/node)
Test cases:
- Single node → [1, 2, 3] for displacement
- Multiple nodes → [1,2,3, 4,5,6, ...] sequential DOFs
- Validates no off-by-one errors
- Validates correct DOF ordering
Ensures DOF mapping used in update_element_cache! is correct.