test(materials): add global material cache test

New 98-line test file included in main test/runtests.jl:
- Tests GlobalMaterialCache construction for stateless and stateful materials
- Tests state access and update operations (get_state, set_state!, update_cache!, reset_cache!)
- Tests state variable helpers (get_state_variable, set_state_variable)
- Tests integration with material traits (create_global_material_cache)
- Validates cache structure and state management for time-dependent materials

Ensures material cache system works correctly for storing state variables at integration points.

test/materials/test_global_material_cache.jl

@@ -0,0 +1,98 @@
+# 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 "Global Material Cache" begin
+    @testset "Basic construction" begin
+        # Test stateless material
+        mat = LinearElastic(E=210e9, ν=0.3)
+        cache = create_global_material_cache(mat, n_ips=4, n_elems=10)
+
+        @test cache isa GlobalMaterialCache
+        @test size(cache.states) == (4, 10)
+        @test size(cache.states_old) == (4, 10)
+
+        # All states should be empty NamedTuple for stateless
+        for elem_id in 1:10, ip in 1:4
+            @test get_state(cache, ip, elem_id) === NamedTuple()
+            @test get_old_state(cache, ip, elem_id) === NamedTuple()
+        end
+    end
+
+    @testset "State access and update" begin
+        # Create a simple stateful cache
+        StateType = NamedTuple{(:κ,), Tuple{Float64}}
+        cache = GlobalMaterialCache{StateType}(2, 3)  # 2 IPs, 3 elements
+
+        # Test initial zeros
+        @test get_state(cache, 1, 1).κ == 0.0
+        @test get_state(cache, 2, 3).κ == 0.0
+
+        # Set a state
+        set_state!(cache, 1, 2, (κ=0.5,))
+        @test get_state(cache, 1, 2).κ == 0.5
+        @test get_old_state(cache, 1, 2).κ == 0.0  # Old unchanged
+
+        # Update cache (copy current → old)
+        update_cache!(cache)
+        @test get_old_state(cache, 1, 2).κ == 0.5  # Now matches current
+
+        # Modify current
+        set_state!(cache, 1, 2, (κ=0.8,))
+        @test get_state(cache, 1, 2).κ == 0.8
+        @test get_old_state(cache, 1, 2).κ == 0.5  # Still from previous step
+
+        # Reset
+        reset_cache!(cache)
+        @test get_state(cache, 1, 2).κ == 0.0
+        @test get_old_state(cache, 1, 2).κ == 0.0
+    end
+
+    @testset "State variable helpers" begin
+        # Create cache with plasticity-like state
+        StateType = NamedTuple{(:ε_p, :α, :κ), Tuple{SymmetricTensor{2,3,Float64,6}, SymmetricTensor{2,3,Float64,6}, Float64}}
+        cache = GlobalMaterialCache{StateType}(1, 1)
+
+        # Set a complex state
+        ε_p_val = SymmetricTensor{2,3}((0.001, 0.0, 0.0, 0.0, 0.0, 0.0))
+        α_val = SymmetricTensor{2,3}((100e6, 0.0, 0.0, 0.0, 0.0, 0.0))
+        κ_val = 0.01
+        state = (ε_p=ε_p_val, α=α_val, κ=κ_val)
+        set_state!(cache, 1, 1, state)
+
+        # Get state and extract variables
+        retrieved_state = get_state(cache, 1, 1)
+        extracted_ε_p = get_state_variable(retrieved_state, PlasticStrain)
+        extracted_α = get_state_variable(retrieved_state, Backstress)
+        extracted_κ = get_state_variable(retrieved_state, EquivalentPlasticStrain)
+
+        @test extracted_ε_p == ε_p_val
+        @test extracted_α == α_val
+        @test extracted_κ == κ_val
+
+        # Test immutable update
+        new_κ = 0.02
+        new_state = set_state_variable(retrieved_state, EquivalentPlasticStrain, new_κ)
+        @test new_state.κ == new_κ
+        @test new_state.ε_p == ε_p_val  # Unchanged
+        @test new_state.α == α_val      # Unchanged
+
+        # Original unchanged (immutable)
+        @test retrieved_state.κ == κ_val
+    end
+
+    @testset "Integration with traits" begin
+        # Verify that cache creation uses material traits correctly
+        mat = LinearElastic(E=210e9, ν=0.3)
+
+        # Material should be stateless
+        @test required_state_variables(mat) === ()
+
+        # Cache should be empty
+        cache = create_global_material_cache(mat, n_ips=2, n_elems=5)
+        @test cache isa GlobalMaterialCache{NamedTuple{(),Tuple{}}}
+    end
+end