test(dofs): add DOF system test

Tests core DOF system functionality.
Validates DOF{T,E} types and DOFSet specifications.

test/dofs/test_dof_system.jl

@@ -0,0 +1,66 @@
+using JuliaFEM
+using Tensors
+
+# Create simple 2-element mesh (2 hexes sharing a face)
+# Hex 1: nodes 1-8
+# Hex 2: nodes 5-8, 9-12 (shares face 5-6-7-8 with Hex 1)
+
+nodes = [
+    Vec{3}((0.0, 0.0, 0.0)),  # 1
+    Vec{3}((1.0, 0.0, 0.0)),  # 2
+    Vec{3}((1.0, 1.0, 0.0)),  # 3
+    Vec{3}((0.0, 1.0, 0.0)),  # 4
+    Vec{3}((0.0, 0.0, 1.0)),  # 5
+    Vec{3}((1.0, 0.0, 1.0)),  # 6
+    Vec{3}((1.0, 1.0, 1.0)),  # 7
+    Vec{3}((0.0, 1.0, 1.0)),  # 8
+    Vec{3}((0.0, 0.0, 2.0)),  # 9
+    Vec{3}((1.0, 0.0, 2.0)),  # 10
+    Vec{3}((1.0, 1.0, 2.0)),  # 11
+    Vec{3}((0.0, 1.0, 2.0)),  # 12
+]
+
+connectivity = [
+    NTuple{8,UInt32}((1, 2, 3, 4, 5, 6, 7, 8)),
+    NTuple{8,UInt32}((5, 6, 7, 8, 9, 10, 11, 12)),
+]
+
+element_sets = Dict{Symbol,Set{UInt32}}(:all => Set(UInt32[1, 2]))
+
+mesh = Mesh{8,Hexahedron{8}}(nodes, connectivity, element_sets)
+
+println("Mesh created:")
+println("  Nodes: ", length(nodes))
+println("  Elements: ", length(connectivity))
+
+# Create elements with 3D displacement DOFs (Vec{3} at vertices)
+println("\nCreating elements with DOF{Vec{3}, Vertex}...")
+
+ElemType = Element{Hexahedron{8}, Lagrange{1}, DOF{Vec{3}, Vertex}}
+elements, mgr = create_elements!(mesh, ElemType)
+
+println("Elements created:")
+println("  Element count: ", length(elements))
+println("  Total DOFs: ", mgr.total_dofs)
+println("  Expected DOFs: ", 3 * length(nodes), " (3 per node)")
+
+# Check DOF indices for first element
+println("\nElement 1 DOF indices:")
+println("  ", elements[1].dof_indices)
+println("  Length: ", length(elements[1].dof_indices))
+println("  Expected: 24 (8 nodes × 3 DOFs)")
+
+# Check DOF indices for second element (should reuse nodes 5-8)
+println("\nElement 2 DOF indices:")
+println("  ", elements[2].dof_indices)
+println("  Length: ", length(elements[2].dof_indices))
+println("  Expected: 24 (8 nodes × 3 DOFs)")
+
+# Verify node-to-DOF mapping
+println("\nNode-to-DOF mapping:")
+for node_id in [1, 5, 9]
+    dofs = get_node_dofs(mgr, node_id)
+    println("  Node $node_id: DOFs $dofs")
+end
+
+println("\n✓ DOF system test complete!")