test(materials): add state variables test

New 93-line test file included in main test/runtests.jl:
- Tests AbstractStateVariable type hierarchy
- Validates state_variable_type trait (returns concrete types)
- Validates default_symbol trait (returns symbols like :ε_p, :α, :κ, :d)
- Tests concrete type instantiation (PlasticStrain, Backstress, etc.)
- Tests NamedTuple usage with state variable symbols
- Verifies trait consistency across all state variable types

Ensures state variable system works correctly for material models.

test/materials/test_state_variables.jl

@@ -0,0 +1,93 @@
+# 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 "State Variables" begin
+    @testset "AbstractStateVariable" begin
+        # Test that state variable types are defined
+        @test PlasticStrain <: AbstractStateVariable
+        @test Backstress <: AbstractStateVariable
+        @test EquivalentPlasticStrain <: AbstractStateVariable
+        @test DamageVariable <: AbstractStateVariable
+    end
+
+    @testset "state_variable_type trait" begin
+        # Test that state_variable_type returns correct concrete types
+        @test state_variable_type(PlasticStrain) === SymmetricTensor{2,3,Float64,6}
+        @test state_variable_type(Backstress) === SymmetricTensor{2,3,Float64,6}
+        @test state_variable_type(EquivalentPlasticStrain) === Float64
+        @test state_variable_type(DamageVariable) === Float64
+    end
+
+    @testset "default_symbol trait" begin
+        # Test that default_symbol returns correct symbols
+        @test default_symbol(PlasticStrain) === :ε_p
+        @test default_symbol(Backstress) === :α
+        @test default_symbol(EquivalentPlasticStrain) === :κ
+        @test default_symbol(DamageVariable) === :d
+    end
+
+    @testset "Concrete type instantiation" begin
+        # Test that we can create instances of the concrete types
+        ε_p_type = state_variable_type(PlasticStrain)
+        ε_p = zero(ε_p_type)
+        @test ε_p isa SymmetricTensor{2,3,Float64,6}
+        @test ε_p == zero(SymmetricTensor{2,3})
+
+        α_type = state_variable_type(Backstress)
+        α = zero(α_type)
+        @test α isa SymmetricTensor{2,3,Float64,6}
+        @test α == zero(SymmetricTensor{2,3})
+
+        κ_type = state_variable_type(EquivalentPlasticStrain)
+        κ = zero(κ_type)
+        @test κ isa Float64
+        @test κ == 0.0
+
+        d_type = state_variable_type(DamageVariable)
+        d = zero(d_type)
+        @test d isa Float64
+        @test d == 0.0
+    end
+
+    @testset "Symbol usage" begin
+        # Test that symbols can be used in NamedTuples
+        ε_p_sym = default_symbol(PlasticStrain)
+        α_sym = default_symbol(Backstress)
+        κ_sym = default_symbol(EquivalentPlasticStrain)
+
+        state = NamedTuple{(ε_p_sym, α_sym, κ_sym)}((
+            zero(SymmetricTensor{2,3}),
+            zero(SymmetricTensor{2,3}),
+            0.0
+        ))
+
+        @test haskey(state, :ε_p)
+        @test haskey(state, :α)
+        @test haskey(state, :κ)
+        @test state.ε_p isa SymmetricTensor{2,3}
+        @test state.α isa SymmetricTensor{2,3}
+        @test state.κ isa Float64
+    end
+
+    @testset "Trait consistency" begin
+        # Verify that all state variables have both traits defined
+        state_vars = [PlasticStrain, Backstress, EquivalentPlasticStrain, DamageVariable]
+
+        for var in state_vars
+            # Should not throw
+            @test state_variable_type(var) !== nothing
+            @test default_symbol(var) !== nothing
+
+            # Symbol should be a Symbol
+            @test default_symbol(var) isa Symbol
+
+            # Type should be a concrete type
+            T = state_variable_type(var)
+            @test isconcretetype(T)
+        end
+    end
+end