test(materials): add material traits test

New 99-line test file included in main test/runtests.jl:
- Tests LinearElastic material traits (supported_physics, required_field_types, required_state_variables)
- Validates trait function existence and exports
- Tests helper functions (is_stateful, get_state_variable_types, get_state_variable_symbols)
- Verifies compositional design philosophy for state variables
- Tests physics-to-field-type derivation

Ensures material trait system works correctly for compositional material design.

test/materials/test_traits.jl

@@ -0,0 +1,99 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+using Test
+using JuliaFEM
+using Tensors
+
+@testset "Material Traits" begin
+    @testset "LinearElastic traits" begin
+        mat = LinearElastic(E=210e9, ν=0.3)
+
+        # Test supported_physics
+        physics = supported_physics(mat)
+        @test physics isa Tuple
+        @test length(physics) == 1
+        @test physics[1] isa Elasticity{3}
+
+        # Test required_field_types (derived from supported_physics)
+        fields = required_field_types(mat)
+        @test fields isa Tuple
+        @test length(fields) == 1
+        @test fields[1] === Displacement{3}
+
+        # Test required_state_variables (stateless)
+        state_vars = required_state_variables(mat)
+        @test state_vars isa Tuple
+        @test length(state_vars) == 0
+        @test isempty(state_vars)
+
+        # Test helper functions
+        @test is_stateful(mat) == false
+
+        state_types = get_state_variable_types(mat)
+        @test state_types isa Tuple
+        @test length(state_types) == 0
+
+        state_symbols = get_state_variable_symbols(mat)
+        @test state_symbols isa Tuple
+        @test length(state_symbols) == 0
+    end
+
+    @testset "Trait function existence" begin
+        # Test that trait functions are exported and callable
+        @test isdefined(JuliaFEM, :supported_physics)
+        @test isdefined(JuliaFEM, :required_field_types)
+        @test isdefined(JuliaFEM, :required_state_variables)
+        @test isdefined(JuliaFEM, :get_state_variable_types)
+        @test isdefined(JuliaFEM, :get_state_variable_symbols)
+        @test isdefined(JuliaFEM, :is_stateful)
+    end
+
+    @testset "Helper function consistency" begin
+        mat = LinearElastic(E=210e9, ν=0.3)
+
+        # For stateless material
+        @test is_stateful(mat) == false
+        @test isempty(required_state_variables(mat))
+        @test isempty(get_state_variable_types(mat))
+        @test isempty(get_state_variable_symbols(mat))
+    end
+
+    @testset "Compositional design verification" begin
+        # This test verifies the compositional design philosophy:
+        # Materials declare tuples of state variable types,
+        # and each state variable has its own traits
+
+        mat = LinearElastic(E=210e9, ν=0.3)
+
+        # LinearElastic should have no state variables
+        vars = required_state_variables(mat)
+        @test vars === ()
+
+        # If a material HAD state variables (e.g., PerfectPlasticity),
+        # we could query each variable's traits like this:
+        # vars = required_state_variables(material)
+        # for var in vars
+        #     concrete_type = state_variable_type(var)
+        #     symbol = default_symbol(var)
+        # end
+
+        # The key insight: state variables are building blocks,
+        # materials compose them to declare requirements
+    end
+
+    @testset "Physics to field type derivation" begin
+        mat = LinearElastic(E=210e9, ν=0.3)
+
+        # Test that required_field_types correctly derives from supported_physics
+        physics = supported_physics(mat)
+        fields = required_field_types(mat)
+
+        @test length(physics) == length(fields)
+
+        for (p, f) in zip(physics, fields)
+            # Each physics should map to its required field
+            @test f === required_field_type(p)
+        end
+    end
+end