feat(mesh,io): stub Gmsh reader and VTU writer without optional deps

Without Gmsh or WriteVTK loaded, read_gmsh_msh and write_vtu_mesh raise clear
ErrorException messages pointing users at the weak extension packages.

- Add gmsh_stub.jl docstrings, read_gmsh_msh stub, mesh_from_current_gmsh_model
- Add write_vtu.jl docstring and write_vtu_mesh stub for WriteVTK extension

src/io/write_vtu.jl

@@ -0,0 +1,40 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+"""
+    write_vtu_mesh(basepath::AbstractString, mesh::Mesh; point_data = (;), cell_data = (;)) -> String
+
+Write a single-block VTU (VTK XML unstructured grid) for `mesh`.
+
+`basepath` must not include a file extension; WriteVTK appends `.vtu`.
+
+# Point and cell fields
+
+- `point_data`: `NamedTuple` of nodal fields. Each value is either a length-`nnodes_total(mesh)`
+  vector (scalar) or a `3 × nnodes_total` matrix (3-vector per node, e.g. displacement).
+- `cell_data`: `NamedTuple` of per-element fields; each value is a length-`nelements(mesh)` vector.
+
+# Weak dependency
+
+This entry point is implemented in `JuliaFEMWriteVTKExt` when
+[`WriteVTK.jl`](https://github.com/JuliaVTK/WriteVTK.jl) is loaded after JuliaFEM:
+
+```julia
+using JuliaFEM, WriteVTK
+write_vtu_mesh(joinpath(outdir, "solution"), mesh; point_data = (; u = uvec))
+```
+
+Supported topologies: linear `Seg2`, `Tri3`, `Quad4`, `Tet4`, and `Hex8` only.
+
+See also: [`read_gmsh_msh`](@ref) for the Gmsh reader extension.
+"""
+function write_vtu_mesh(args...; kwargs...)
+    throw(
+        ErrorException(
+            "write_vtu_mesh requires the WriteVTK.jl package. " *
+            "Add it to your environment (`import Pkg; Pkg.add(\"WriteVTK\")`) " *
+            "and run `using WriteVTK` after `using JuliaFEM` so the " *
+            "`JuliaFEMWriteVTKExt` extension can load.",
+        ),
+    )
+end

src/mesh/gmsh_stub.jl

@@ -0,0 +1,48 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+"""
+    read_gmsh_msh(path::AbstractString; kwargs...) -> Mesh
+
+Read a Gmsh `.msh` file into a concrete [`Mesh`](@ref).
+
+Supported volume/surface Gmsh element types (linear only): **3-node triangle** (2),
+**4-node quadrilateral** (3), **4-node tetrahedron** (4), **8-node hexahedron** (5).
+Mixed cell types in the same file are not supported.
+
+Keyword `dim` may be `2` or `3` to select surface or volume elements; the default
+chooses dimension **3** if the model has 3D cells, otherwise **2**.
+
+Physical groups on the mesh dimension populate [`element_sets`](@ref) using
+sanitized physical names (fallback `physical_{dim}_{tag}`). Physical groups one
+dimension lower (boundary curves in 2D, boundary faces in 3D) populate
+[`node_sets`](@ref).
+
+Implemented in `JuliaFEMGmshExt` when [`Gmsh.jl`](https://github.com/JuliaFEM/Gmsh.jl)
+is loaded after `using JuliaFEM` (`import Pkg; Pkg.add("Gmsh")` then `using Gmsh`).
+Without that package, calling `read_gmsh_msh` with a path string raises an
+`ErrorException` with install instructions (other arities always raise here).
+"""
+function read_gmsh_msh(args...; kwargs...)
+    throw(
+        ErrorException(
+            "read_gmsh_msh requires the Gmsh.jl package. " *
+            "Add it to your environment (`import Pkg; Pkg.add(\"Gmsh\")`) " *
+            "and run `using Gmsh` after `using JuliaFEM` so the " *
+            "`JuliaFEMGmshExt` extension can load.",
+        ),
+    )
+end
+
+"""
+    mesh_from_current_gmsh_model(; kwargs...) -> Mesh
+
+Build a [`Mesh`](@ref) from the **currently active** Gmsh model (after
+`Gmsh.initialize`, geometry, and `gmsh.model.mesh.generate`).
+
+The same element-type rules and keyword `dim` apply as for [`read_gmsh_msh`](@ref).
+
+Implemented in `JuliaFEMGmshExt` when `Gmsh` is loaded. Without it, you get a
+`MethodError` (no method defined).
+"""
+function mesh_from_current_gmsh_model end