feat(materials): add hydraulic conductivity for Darcy flow

Wire `HydraulicConductivity` into potential-based Darcy kernels.

- Provide permeability-driven flux linearizations with documented traits.

src/materials/hydraulic_conductivity.jl

@@ -0,0 +1,78 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Linear isotropic hydraulic conductivity for the **primal** flow potential
+(Darcy) equation in steady state,
+
+    q = -K · ∇p,    ∇ · q = f   ⇒   -∇ · (K · ∇p) = f,
+
+with scalar potential `p` (pressure head) discretized like temperature.
+The weak stiffness `∫ ∇N_i · K · ∇N_j dV` is assembled by [`HeatKernel`](@ref)
+via [`DarcyPotentialKernel`](@ref); see `src/domains/darcy/potential.jl`.
+
+For mixed RT₀–P₀ Darcy ([`DarcyMixedRT0P0Kernel`](@ref), [`DarcyMixedHex8RT0P0Kernel`](@ref)),
+[`HydraulicConductivity`](@ref) supplies isotropic `K` → `inv_K = (1/K) I`; pass a symmetric tensor
+directly to those kernels for anisotropic inverse conductivity. This struct still supplies `K`
+for the primal potential path via [`DarcyPotentialKernel`](@ref).
+
+# Fields
+- `K::Float64` — isotropic hydraulic conductivity [m/s]; must be positive.
+
+# Example
+```julia
+soil = HydraulicConductivity(K = 1e-4)
+kernel = DarcyPotentialKernel(ContinuumFormulation{FullThreeD}(), soil)
+```
+"""
+
+using Tensors
+
+struct HydraulicConductivity <: AbstractMaterial
+    K::Float64
+
+    function HydraulicConductivity(K::Float64)
+        K > 0.0 || throw(ArgumentError("Hydraulic conductivity K must be positive, got K = $K"))
+        new(K)
+    end
+end
+
+HydraulicConductivity(; K) = HydraulicConductivity(Float64(K))
+
+material_behavior(::HydraulicConductivity) = StatelessConstantTangent()
+supported_physics(::HydraulicConductivity) = (Thermal{3}(),)
+required_state_variables(::HydraulicConductivity) = ()
+
+"""
+    hydraulic_conductivity_tensor(material::HydraulicConductivity)
+
+Isotropic tensor `K · I` used at quadrature points (same storage pattern as
+[`conductivity_tensor`](@ref) for [`HeatConductivity`](@ref)).
+"""
+@inline function hydraulic_conductivity_tensor(material::HydraulicConductivity)
+    return material.K * one(SymmetricTensor{2,3,Float64,6})
+end
+
+@inline scalar_diffusion_tensor(material::HydraulicConductivity) =
+    hydraulic_conductivity_tensor(material)
+
+"""
+    compute_seepage_flux(material::HydraulicConductivity, ∇p, state_old, Δt)
+        -> (q, Ktensor, state_new)
+
+Darcy analogue of [`compute_heat_flux`](@ref): `q = -K · ∇p`, tangent `Ktensor`
+is `∂q/∂(∇p)` up to the same sign convention as the heat routine.
+"""
+function compute_seepage_flux(
+    material::HydraulicConductivity,
+    ∇p::Vec{3,T},
+    state_old::Union{Nothing, NamedTuple},
+    Δt::Float64,
+) where {T}
+    Ktensor = hydraulic_conductivity_tensor(material)
+    q = -Ktensor ⋅ ∇p
+    return q, Ktensor, NamedTuple()
+end
+
+compute_seepage_flux(material::HydraulicConductivity, ∇p::Vec{3,T}) where {T} =
+    compute_seepage_flux(material, ∇p, nothing, 0.0)