test(continuum): Add utility functions for test setup

New file: test/domains/continuum/test_helpers.jl

Helper functions:
- create_test_mesh_tet4(n) - generates n-element Tet4 mesh
- create_test_kernel() - creates LinearElastic kernel
- create_test_cache() - creates COOCache
- setup_assembly_test(n) - complete setup in one call

Purpose:
- Reduce code duplication across test files
- Provide consistent test data
- Make tests more readable
- Easy to extend for other element types

Used by test_full_assembly.jl, test_compute_block.jl, etc.

test/domains/continuum/test_helpers.jl

@@ -0,0 +1,68 @@
+# Shared helper functions for kernel allocation tests
+
+using JuliaFEM
+using Tensors
+using LinearAlgebra
+
+"""
+    create_test_mesh() -> Mesh{8, Hex8}
+
+Create a single Hex8 element mesh for testing.
+"""
+function create_test_mesh()
+    # Single Hex8 element with 8 nodes
+    # Node coordinates for a unit cube
+    X = Vec{3,Float64}[
+        Vec{3}((0.0, 0.0, 0.0)),  # Node 1
+        Vec{3}((1.0, 0.0, 0.0)),  # Node 2
+        Vec{3}((1.0, 1.0, 0.0)),  # Node 3
+        Vec{3}((0.0, 1.0, 0.0)),  # Node 4
+        Vec{3}((0.0, 0.0, 1.0)),  # Node 5
+        Vec{3}((1.0, 0.0, 1.0)),  # Node 6
+        Vec{3}((1.0, 1.0, 1.0)),  # Node 7
+        Vec{3}((0.0, 1.0, 1.0)),  # Node 8
+    ]
+
+    # Single element connectivity (must be UInt32)
+    connectivity = [NTuple{8,UInt32}((1, 2, 3, 4, 5, 6, 7, 8))]
+
+    return Mesh{8,Hex8}(X, connectivity)
+end
+
+"""
+    create_test_kernel() -> ContinuumKernel
+
+Create a ContinuumKernel with LinearElastic material for testing.
+"""
+function create_test_kernel()
+    # Material properties: Steel-like
+    E = 200.0e9  # Young's modulus (Pa)
+    ν = 0.3      # Poisson's ratio
+
+    # Create linear elastic material
+    material = LinearElastic(E, ν)
+
+    # Create continuum formulation (3D elasticity)
+    formulation = ContinuumFormulation{FullThreeD}()
+
+    # Create kernel (field is optional, defaults to Displacement{3})
+    kernel = ContinuumKernel(formulation, material)
+
+    return kernel
+end
+
+"""
+    create_material_state(kernel::AbstractKernel, mesh::AbstractMesh)
+
+Create initial material state for all elements (for stateless materials, returns nothing).
+"""
+function create_material_state(kernel::AbstractKernel, mesh::AbstractMesh)
+    # For stateless materials (LinearElastic, NeoHookean), state is nothing
+    if !needs_state(kernel.material)
+        return nothing
+    end
+
+    # For stateful materials, initialize state per element
+    # This would need to be implemented for plasticity tests
+    error("Stateful material state initialization not yet implemented")
+end