test(continuum): Add tests for DOF mapping functions

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.

test/domains/continuum/test_dof_mapping.jl

@@ -0,0 +1,23 @@
+# Test get_dof_mapping! function
+
+@testset "get_dof_mapping!" begin
+    kernel = create_test_kernel()
+    mesh = create_test_mesh()
+
+    # Allocate DOF buffer (8 nodes * 3 DOFs = 24)
+    dofs = zeros(Int, 24)
+
+    # Test DOF mapping for element 1
+    get_dof_mapping!(dofs, kernel, 1, mesh)
+
+    # Verify correct DOF indices (1-indexed)
+    # For element 1 with nodes [1,2,3,4,5,6,7,8]:
+    # Node 1: DOFs [1,2,3], Node 2: DOFs [4,5,6], etc.
+    expected_dofs = collect(1:24)
+    @test dofs == expected_dofs
+
+    # Test zero allocations (warm-up call first)
+    get_dof_mapping!(dofs, kernel, 1, mesh)
+    allocs = @allocated get_dof_mapping!(dofs, kernel, 1, mesh)
+    @test allocs == 0
+end