test(continuum): Add tests for dofs_per_node function

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.

test/domains/continuum/test_dofs_per_node.jl

@@ -0,0 +1,12 @@
+# Test dofs_per_node function
+
+@testset "dofs_per_node" begin
+    kernel = create_test_kernel()
+
+    # Test that it returns 3 for 3D displacement field
+    @test dofs_per_node(kernel) == 3
+
+    # Test zero allocations
+    allocs = @allocated dofs_per_node(kernel)
+    @test allocs == 0
+end