feat(materials): add moisture diffusion conductivity tag

Introduce `MoistureDiffusivity` for scalar moisture transport kernels.

- Declare physics traits and constant diffusion tensors/scalars as appropriate.

src/materials/moisture_diffusivity.jl

@@ -0,0 +1,27 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+"""
+Isotropic moisture diffusivity `D_w` for a scalar humidity/pore-water field
+[`MoistureContent`](@ref). The weak operator matches steady heat conduction
+(`HeatKernel` with [`Thermal`](@ref) physics).
+"""
+
+using Tensors
+
+struct MoistureDiffusivity <: AbstractMaterial
+    D_w::Float64
+
+    function MoistureDiffusivity(D_w::Float64)
+        D_w > 0 || throw(ArgumentError("Moisture diffusivity D_w must be positive"))
+        new(D_w)
+    end
+end
+
+MoistureDiffusivity(; D_w::Real) = MoistureDiffusivity(Float64(D_w))
+
+material_behavior(::MoistureDiffusivity) = StatelessConstantTangent()
+supported_physics(::MoistureDiffusivity) = (Thermal{3}(),)
+
+@inline conductivity_tensor(m::MoistureDiffusivity) = m.D_w * one(SymmetricTensor{2,3,Float64,6})
+@inline scalar_diffusion_tensor(m::MoistureDiffusivity) = conductivity_tensor(m)