feat(materials): add eigenstrain-augmented linear elasticity

Support prescribed eigenstrain tensors updated outside `compute_stress`.

- Add `LinearElasticWithEigenstrain` and matching state-variable declarations.

src/materials/eigenstrain_elastic.jl

@@ -0,0 +1,40 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+"""
+Linear elastic response on the mechanical strain `ε − ε_e`, where the eigenstrain
+`ε_e` (e.g. drying shrinkage) is stored per integration point and updated outside this
+routine—typically after a moisture diffusion solve.
+"""
+
+using Tensors
+
+struct LinearElasticWithEigenstrain <: AbstractElasticMaterial
+    elastic::LinearElastic
+end
+
+LinearElasticWithEigenstrain(; E::Real, ν::Real) = LinearElasticWithEigenstrain(LinearElastic(E = E, ν = ν))
+
+material_behavior(::LinearElasticWithEigenstrain) = StatefulStrainDependent()
+supported_physics(::LinearElasticWithEigenstrain) = (Elasticity{3}(),)
+required_state_variables(::LinearElasticWithEigenstrain) = (Eigenstrain,)
+
+function compute_stress(
+    mat::LinearElasticWithEigenstrain,
+    ε::SymmetricTensor{2,3},
+    ::Nothing,
+    Δt::Float64,
+)
+    return compute_stress(mat, ε, NamedTuple(), Δt)
+end
+
+function compute_stress(
+    mat::LinearElasticWithEigenstrain,
+    ε::SymmetricTensor{2,3},
+    state_old::NamedTuple,
+    Δt::Float64,
+)
+    ε_e = get(state_old, :ε_e, zero(SymmetricTensor{2,3}))
+    σ, 𝔻, _ = compute_stress(mat.elastic, ε - ε_e, NamedTuple(), 0.0)
+    return σ, 𝔻, (ε_e=ε_e,)
+end