docs(io,mesh): describe Gmsh and VTK extension workflows

Align module READMEs with the weakdep extension pattern: WriteVTK VTU export,
Gmsh MSH import, and thin stubs in the unconditional load path.

- Expand src/io/README with SPDX banner, VTK section, and stub policy note
- Mesh README import blurb cites Gmsh stubs and JuliaFEMGmshExt activation

src/io/README.md

@@ -1,19 +1,37 @@
+<!--
+SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+SPDX-License-Identifier: MIT
+-->
+
 # src/io/
 
-Policy for **external mesh formats** (Gmsh, Netgen, Abaqus, Code Aster, …).
+Policy for **external mesh formats** and **visualisation export** (Gmsh, VTK, …).
 
 The core package owns the type-stable mesh model (`Mesh{N, T}` under
-`src/mesh/`) and assembly; it does **not** ship format-specific parsers in
-`src/io/`. Converters that read third-party files and build `Mesh{…}`,
-`element_sets`, and `node_sets` should live in **separate packages** or in
-**JuliaFEM package extensions** wired through `[weakdeps]` / `[extensions]` in
-`Project.toml`, mirroring the MPI extension pattern (`JuliaFEMMPIExt`).
+`src/mesh/`) and assembly; it does **not** ship format-specific parsers or writers
+in unconditional `src/io/*.jl` runtime paths. Converters live in **package
+extensions** wired through `[weakdeps]` / `[extensions]` in `Project.toml`, mirroring
+the MPI pattern (`JuliaFEMMPIExt`).
 
 Legacy `.inp` / `.med` readers tied to the pre-reset API remain under
 `src/legacy/io/` and load only with `JULIAFEM_ENABLE_LEGACY=1`.
 
+## VTK (WriteVTK)
+
+- Weak dependency: `WriteVTK` → extension `JuliaFEMWriteVTKExt`.
+- User workflow: `using JuliaFEM, WriteVTK` then [`write_vtu_mesh`](@ref) to write
+  `.vtu` unstructured grids with optional nodal (`point_data`) and per-element
+  (`cell_data`) fields.
+- Supported element shapes for export: linear `Seg2`, `Tri3`, `Quad4`, `Tet4`,
+  `Hex8` (same single-topology assumption as the rest of the mesh stack).
+
+## Gmsh
+
+- See [`read_gmsh_msh`](@ref) (`JuliaFEMGmshExt`).
+
 ## This directory
 
-This folder holds **documentation only** until a new extension or companion
-package is added. Do not grow unconditional `include("io/*.jl")` paths in
-`JuliaFEM.jl` for heavy or format-specific I/O.
+`write_vtu.jl` defines the [`write_vtu_mesh`](@ref) stub and docstring; the
+implementation is in `ext/JuliaFEMWriteVTKExt.jl`. Do not add heavy or
+format-specific I/O to unconditional `JuliaFEM.jl` includes beyond thin stubs
+like this.

src/mesh/README.md

@@ -69,8 +69,11 @@ longest edge of each element, doubling the element count per level.
 
 Mesh import from external tools (Gmsh, Netgen, Abaqus, …) is **not** part of
 the core load path; use optional extensions or companion packages that build
-`Mesh{…}` (see `src/io/README.md`). VTK / XDMF output is not implemented in the
-new path; legacy results writers under `src/legacy/` cover older tests.
+`Mesh{…}` (see `src/io/README.md`). For **Gmsh**, load `Gmsh.jl` after JuliaFEM
+so `JuliaFEMGmshExt` activates, then call [`read_gmsh_msh`](@ref) or
+[`mesh_from_current_gmsh_model`](@ref) (`src/mesh/gmsh_stub.jl`). VTK / XDMF
+output is not implemented in the new path; legacy results writers under
+`src/legacy/` cover older tests.
 
 ## Related code