test(dofs): add multicomponent facet DOF tests

test/dofs/test_multicomponent_facet_dofs.jl

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+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+using Test
+using LinearAlgebra
+using JuliaFEM
+using JuliaFEM: FacetMassKernel, EdgeMassKernel
+using JuliaFEM: DOFBasedCOOAssembler, DOFBasedCOOCache, assemble!, extract_system
+
+# Reference Tet4 from `test_tet4_facet_maps.jl`.
+function _reference_tet4_mesh()
+    nodes = Vec{3, Float64}[
+        Vec(0.0, 0.0, 0.0),
+        Vec(1.0, 0.0, 0.0),
+        Vec(0.0, 1.0, 0.0),
+        Vec(0.0, 0.0, 1.0),
+    ]
+    conn = (UInt32(1), UInt32(2), UInt32(3), UInt32(4))
+    return Mesh{Tet4}(nodes, [conn]), nodes
+end
+
+@testset "DOFHandler Vec{2} on Edge — two globals per topological edge" begin
+    mesh, _ = _reference_tet4_mesh()
+    S = @DOFSet{circ::DOF{Vec{2, Float64}, Edge}}
+    elements, handler = create_elements!(mesh, Element{Tet4, Lagrange{1}, S})
+    @test handler.total_dofs == 12
+    @test length(elements) == 1
+    @test length(elements[1].dof_indices) == 12
+end
+
+@testset "EdgeMassKernel diagonal — Vec{2} replicates scalar edge measure per component" begin
+    mesh, nodes = _reference_tet4_mesh()
+    X = nodes
+    expected_edge_len = sum(tet_edge_length_physical(X, le) for le in 1:6)
+
+    S = @DOFSet{circ::DOF{Vec{2, Float64}, Edge}}
+    elements, handler = create_elements!(mesh, Element{Tet4, Lagrange{1}, S})
+    kernel = EdgeMassKernel(mesh)
+    cache = DOFBasedCOOCache(elements, handler, mesh, kernel)
+    assemble!(cache, DOFBasedCOOAssembler(), kernel, mesh)
+    K, _ = extract_system(cache)
+    @test sum(diag(Matrix(K))) ≈ 2 * expected_edge_len rtol = 1e-10
+end
+
+@testset "FacetMassKernel diagonal — Vec{2} on Face (Hex8 unit cube)" begin
+    mesh = create_unit_cube_mesh(Hex8; nx=1, ny=1, nz=1)
+    @test nelements(mesh) == 1
+    conn = mesh.connectivity[1]
+    X = Vec{3, Float64}[mesh.nodes[Int(conn[i])] for i in 1:8]
+    expected_face_area = sum(hex8_face_area_physical(X, lf) for lf in 1:6)
+    @test expected_face_area ≈ 6.0 rtol = 1e-12
+
+    S = @DOFSet{flux::DOF{Vec{2, Float64}, Face}}
+    elements, handler = create_elements!(mesh, Element{Hex8, Lagrange{1}, S})
+    kernel = FacetMassKernel(mesh)
+    cache = DOFBasedCOOCache(elements, handler, mesh, kernel)
+    assemble!(cache, DOFBasedCOOAssembler(), kernel, mesh)
+    K, _ = extract_system(cache)
+    @test sum(diag(Matrix(K))) ≈ 2 * expected_face_area rtol = 1e-10
+end
+
+@testset "FacetMassKernel diagonal — Vec{2} on Face (Tet4)" begin
+    mesh, nodes = _reference_tet4_mesh()
+    X = nodes
+    expected_face_area = sum(tet_face_area_physical(X, lf) for lf in 1:4)
+
+    S = @DOFSet{flux::DOF{Vec{2, Float64}, Face}}
+    elements, handler = create_elements!(mesh, Element{Tet4, Lagrange{1}, S})
+    kernel = FacetMassKernel(mesh)
+    cache = DOFBasedCOOCache(elements, handler, mesh, kernel)
+    assemble!(cache, DOFBasedCOOAssembler(), kernel, mesh)
+    K, _ = extract_system(cache)
+    @test sum(diag(Matrix(K))) ≈ 2 * expected_face_area rtol = 1e-10
+end