refactor(domains): Darcy, heat, and thermo-elastic kernels

Align primal Darcy and heat kernels with Pass~1 updates; extend thermo_elastic kernel; document new poroelastic/THM folders in domains README.

src/domains/README.md

@@ -1,3 +1,8 @@
+<!--
+SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+SPDX-License-Identifier: MIT
+-->
+
 # src/domains/
 
 Physics kernels that plug into the shared assembler infrastructure. Each
@@ -17,6 +22,8 @@ same checkout.
 | `heat/` | `HeatKernel` | `test/domains/heat/` |
 | `darcy/` | `DarcyPotentialKernel` (primal potential); `DarcyMixedRT0P0Kernel` (Tet4 RT₀–P₀ H(div)) | `test/domains/darcy/` |
 | `thermo_elastic/` | `ThermoElasticKernel` (multi-field `u` + `T`) | `test/domains/thermo_elastic/` |
+| `poroelastic/` | `BiotPoroelasticKernel` (multi-field `u` + pore pressure `p`) | `test/domains/poroelastic/` |
+| `thermo_poroelastic/` | `ThermoPoroelasticKernel` (`u` + `T` + `p`; optional `kappa_tp`, `zeta_tp`, `heat_capacity`, `density` for `M_uu`). Module notes: `thermo_poroelastic/README.md`. | `test/domains/thermo_poroelastic/` (incl. column thermal decay vs Fourier) |
 
 Narrative walkthrough for thermo-elasticity: `docs/src/thermo_elastic_walkthrough.md`
 in the package Documenter tree.

src/domains/darcy/potential.jl

@@ -30,7 +30,7 @@ steady primal Darcy: same assembly path as thermal diffusion with tensor
 
 ```julia
 kernel = DarcyPotentialKernel(
-    ContinuumFormulation{FullThreeD}(),
+    ContinuumFormulation{ThreeDimensional}(),
     HydraulicConductivity(K = 1e-4),
 )
 ```

src/domains/heat/kernel.jl

@@ -36,7 +36,7 @@ Differences from `ContinuumKernel`:
 using Tensors
 
 using ..JuliaFEM: AbstractKernel, AbstractFormulation, AbstractMaterial, AbstractField
-using ..JuliaFEM: ContinuumFormulation, FullThreeD
+using ..JuliaFEM: ContinuumFormulation, ThreeDimensional
 using ..JuliaFEM: Temperature, MoistureContent, PressurePotential, dofs_per_node, get_field
 using ..JuliaFEM: HeatConductivity, MoistureDiffusivity, HydraulicConductivity,
     ElementWiseScalarDiffusion, scalar_diffusion_tensor
@@ -58,7 +58,7 @@ Domain kernel for steady-state **scalar diffusion**: linear heat conduction
 volume element.
 
 Reuses `ContinuumFormulation{Theory}` (the geometric formulation is
-field-agnostic; `FullThreeD` works equally well for displacement,
+field-agnostic; `ThreeDimensional` works equally well for displacement,
 temperature, potential, …).
 
 # Fields
@@ -71,7 +71,7 @@ temperature, potential, …).
 
 ```julia
 kernel = HeatKernel(
-    ContinuumFormulation{FullThreeD}(),
+    ContinuumFormulation{ThreeDimensional}(),
     HeatConductivity(k = 401.0),
 )
 ```

src/domains/thermo_elastic/kernel.jl

@@ -41,7 +41,7 @@ geometry batches via `GeometryCache.∇N_data` / `GeometryCache.N_data`
 using Tensors
 
 using ..JuliaFEM: AbstractKernel, AbstractFormulation
-using ..JuliaFEM: ContinuumFormulation, FullThreeD, AbstractContinuumTheory
+using ..JuliaFEM: ContinuumFormulation, ThreeDimensional, AbstractContinuumTheory
 using ..JuliaFEM: AbstractMaterial, LinearElastic, HeatConductivity
 using ..JuliaFEM: elasticity_tensor, conductivity_tensor
 using ..JuliaFEM: AssemblyMaterialWorkspace