feat(domains): add DarcyPotentialKernel wrapping HeatKernel

Expose steady primal Darcy head formulation as thermal diffusion with
`HydraulicConductivity` and `PressurePotential` fields on shared assembler paths.

src/domains/darcy/potential.jl

@@ -0,0 +1,52 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Primal **Darcy flow potential** on a conforming scalar vertex field.
+
+Steady saturated flow with hydraulic head `p` obeys
+
+``-\\nabla \\cdot (K \\nabla p) = f``
+
+(weak form identical to steady heat conduction with conductivity `K`).
+[`DarcyPotentialKernel`](@ref) is a thin constructor around [`HeatKernel`](@ref)
+with [`HydraulicConductivity`](@ref) and [`PressurePotential`](@ref).
+
+Mixed lowest-order H(div) flux + cell pressure on Tet4 lives in
+[`DarcyMixedRT0P0Kernel`](@ref) (`mixed_rt0.jl`). Transient storage is still future work.
+"""
+
+using ..JuliaFEM: ContinuumFormulation, HeatKernel, HydraulicConductivity,
+    ElementWiseScalarDiffusion, PressurePotential
+
+"""
+    DarcyPotentialKernel(formulation, material[, field])
+
+Return [`HeatKernel`](@ref)`{…, HydraulicConductivity, PressurePotential}` for
+steady primal Darcy: same assembly path as thermal diffusion with tensor
+[`hydraulic_conductivity_tensor`](@ref).
+
+# Example
+
+```julia
+kernel = DarcyPotentialKernel(
+    ContinuumFormulation{FullThreeD}(),
+    HydraulicConductivity(K = 1e-4),
+)
+```
+"""
+function DarcyPotentialKernel(
+    formulation::ContinuumFormulation{Theory},
+    material::HydraulicConductivity,
+    field::PressurePotential = PressurePotential(),
+) where {Theory}
+    return HeatKernel(formulation, material, field; heat_capacity = 0.0)
+end
+
+function DarcyPotentialKernel(
+    formulation::ContinuumFormulation{Theory},
+    material::ElementWiseScalarDiffusion,
+    field::PressurePotential = PressurePotential(),
+) where {Theory}
+    return HeatKernel(formulation, material, field; heat_capacity = 0.0)
+end