test(basis): add zero-cost basis test

Tests zero-allocation basis function evaluation.
Validates performance characteristics of basis functions.

test/basis/test_nbasis_zerocost.jl

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+# Test that nbasis functions are zero-cost (compile to constants)
+
+using Test
+using JuliaFEM
+
+@testset "nbasis API" begin
+    # Test that nbasis is exported and works
+    @test nbasis(Triangle{3}(), Lagrange{1}()) == 3
+    @test nbasis(Triangle{6}(), Lagrange{2}()) == 6
+    @test nbasis(Tetrahedron{4}(), Lagrange{1}()) == 4
+    @test nbasis(Tetrahedron{10}(), Lagrange{2}()) == 10
+    @test nbasis(Hexahedron{8}(), Lagrange{1}()) == 8
+    @test nbasis(Hexahedron{27}(), Lagrange{2}()) == 27
+    
+    # Serendipity families
+    @test nbasis(Quadrilateral{8}(), Serendipity{2}()) == 8
+    @test nbasis(Hexahedron{20}(), Serendipity{2}()) == 20
+end
+
+@testset "nbasis is zero-cost" begin
+    # Verify that nbasis compiles to a constant
+    # We use @allocated to check - should be 0 bytes
+    
+    @test (@allocated nbasis(Triangle{3}(), Lagrange{1}())) == 0
+    @test (@allocated nbasis(Tetrahedron{10}(), Lagrange{2}())) == 0
+    @test (@allocated nbasis(Quadrilateral{8}(), Serendipity{2}())) == 0
+    
+    # Also verify with @code_llvm that it returns a constant
+    # (This would be done manually during development)
+    # @code_llvm nbasis(Triangle{3}(), Lagrange{1}())  # Should show: ret i64 3
+end
+
+@testset "validate_dof_consistency" begin
+    # Test that validation works correctly
+    
+    # Valid cases (should not throw)
+    @test validate_dof_consistency(ScalarDOF(), Triangle{3}, Lagrange{1}) === nothing
+    
+    # Invalid cases: wrong number of DOFs (should throw ArgumentError)
+    @test_throws ArgumentError validate_dof_consistency(
+        VectorDOF{2}(),  # 2 components per node × 3 nodes = 6 DOFs
+        Triangle{3},     # 3 nodes
+        Lagrange{1}      # 3 basis functions
+        # Mismatch: 6 DOFs ≠ 3 basis functions
+    )
+    
+    @test_throws ArgumentError validate_dof_consistency(
+        VectorDOF{3}(),    # 3 components per node × 10 nodes = 30 DOFs
+        Tetrahedron{10},   # 10 nodes
+        Lagrange{2}        # 10 basis functions
+        # Mismatch: 30 DOFs ≠ 10 basis functions
+    )
+    
+    # Test error message content for one case
+    ex = try
+        validate_dof_consistency(VectorDOF{2}(), Triangle{3}, Lagrange{1})
+        nothing
+    catch e
+        e
+    end
+    @test ex isa ArgumentError
+    @test occursin("3 basis functions", ex.msg)
+    @test occursin("6 DOFs", ex.msg)
+    @test occursin("Ciarlet triplet", ex.msg)
+end
+
+@testset "nbasis consistency with get_basis_functions" begin
+    # Verify that nbasis returns the same count as actual basis functions
+    
+    using StaticArrays: SVector
+    using Tensors: Vec
+    
+    test_cases = [
+        (Lagrange{1}, Triangle{3}, Vec(0.25, 0.25)),
+        (Lagrange{2}, Triangle{6}, Vec(1/3, 1/3)),
+        (Lagrange{1}, Tetrahedron{4}, Vec(0.25, 0.25, 0.25)),
+        (Lagrange{1}, Hexahedron{8}, Vec(0.0, 0.0, 0.0)),
+    ]
+    
+    for (basis_type, topo_type, xi) in test_cases
+        topo = topo_type()
+        basis = basis_type()
+        
+        # Get actual basis function count
+        N_actual = get_basis_functions(topo, basis, xi)
+        n_actual = length(N_actual)
+        
+        # Get count from nbasis
+        n_expected = nbasis(topo, basis)
+        
+        @test n_actual == n_expected
+    end
+end