docs(src): align module map with extension-first mesh I/O

Add SPDX to src/README, trim mesh/io blurbs for extension-based formats, and
extend thermo-poroelastic notes with Val-split THM helpers and a test pointer.

- SPDX HTML banner on src/README plus refreshed mesh and io table descriptions
- Document `_thm_stiff_K_*` / `_thm_mass_M_*` helpers and microkernel alloc test

src/README.md

@@ -1,3 +1,8 @@
+<!--
+SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+SPDX-License-Identifier: MIT
+-->
+
 # src/
 
 Top-level layout of the JuliaFEM source tree. The package is **0.x**; the
@@ -17,14 +22,14 @@ inside each subdirectory.
 | `basis/`        | Shape-function families (`Lagrange{P}`, `Serendipity{P}`, plate bases) plus the symbolic generator that produces them.       |
 | `quadrature/`   | Gauss-Legendre integration points for every supported topology.                                                              |
 | `geometry/`     | Jacobians, physical derivatives, strain tensors.                                                                             |
-| `mesh/`         | Type-stable `Mesh{N, Topo}`, structured/circular generators, refinement, RCM ordering, gmsh wrapper.                         |
+| `mesh/`         | Type-stable `Mesh{N, Topo}`, structured/circular generators, refinement, RCM ordering.                                      |
 | `dofs/`         | `DOF{Quantity, Entity}`, `@DOFSet`, `DOFHandler`, DOF connectivity and the field/dof-extraction infrastructure.              |
 | `elements/`     | `Element{K, P, S, N}` template, compile-time `local_dof_layout`, DOF-extraction and field-interpolation utilities.           |
 | `materials/`    | `LinearElastic`, `NeoHookean`, `PerfectPlasticity`, `HeatConductivity` and the trait-based dispatch used by the kernels.     |
 | `assemblers/`   | Assembly caches, `ElementBasedAssembler` (COO/CSC) and `DOFBasedCOOAssembler`, the matrix-free `apply_K!`/`apply_M!` path.   |
 | `domains/`      | Physics kernels per discipline: `continuum/`, `heat/`, `thermo_elastic/`. See `domains/README.md`; each subdirectory implements an `AbstractKernel`. |
 | `physics/`      | `AbstractPhysics` tag (`Elasticity{Dim}`, `Thermal{Dim}`), microkernel contract for the DOF-based path, formulation helpers and strain extraction. |
-| `io/`           | Mesh readers (Gmsh by default; Abaqus + Aster live in the optional `Legacy` submodule).                                      |
+| `io/`           | Mesh I/O policy only; external formats via extensions / side packages; Abaqus + Aster in optional `Legacy`.                 |
 | `sparse/`       | Lightweight sparse-matrix utilities (DOK / COO).                                                                             |
 | `fields/`       | `AbstractField` (`Displacement`, `Temperature`, `DisplacementRotation`) and the `LocalField` interpolation type.             |
 

src/domains/thermo_poroelastic/README.md

@@ -14,6 +14,10 @@ optional cross-coupling between `T` and `p` (`kappa_tp`, `zeta_tp`). See
 `α`, thermo-elastic `M_Tu` / `M_uT` from `β`, optional `M_uu` from solid
 `density`).
 
+Stiffness and mass microkernels split non-zero `(field_i, field_j)` blocks into
+small `Val`-tagged helpers (`_thm_stiff_K_*`, `_thm_mass_M_*`) for clearer IR;
+see `test_thm_microkernel_alloc.jl` for per-pair `@allocated` checks.
+
 Monolithic transient stepping (backward Euler on `(K + M/Δt)`) can use the same
 `DOFBasedCOOCache` with `assemble!` + `assemble_M!`.