feat(docs): add thermo_elastic_walkthrough.md

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docs/src/thermo_elastic_walkthrough.md

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+# [Thermo-elasticity: multi-field walkthrough](@id thermo_elastic_walkthrough)
+
+This page is a **narrative companion** to
+[`test/domains/thermo_elastic/test_thermo_elastic_kernel.jl`](https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/test/domains/thermo_elastic/test_thermo_elastic_kernel.jl),
+which is the executable contract for the coupled `u` + `T` path through the
+DOF-based assembler.
+
+## What you assemble
+
+`ThermoElasticKernel` couples vertex displacement `u` (3 DOFs per node) with
+vertex temperature `T` (1 DOF per node). The global stiffness has blocks
+`K_uu`, `K_TT`, and (when the coupling coefficient `β ≠ 0`) off-diagonal
+`K_uT` / `K_Tu`. With `β = 0` the kernel is block-diagonal and matches
+independent elasticity and heat solves on the same mesh.
+
+## Minimal code (block-diagonal smoke)
+
+The following uses the same material stack as the regression test, a small
+`Hex8` box, and `DOFBasedCOOCache` (the multi-field kernel is wired through this
+cache type rather than the narrow `create_cache` overload used for
+`ContinuumKernel` alone).
+
+```julia
+using JuliaFEM
+using SparseArrays
+
+mesh = create_structured_box_mesh(Hex8;
+    xmin = 0.0, xmax = 1.0, nx = 2,
+    ymin = 0.0, ymax = 1.0, ny = 2,
+    zmin = 0.0, zmax = 1.0, nz = 2,
+)
+
+mech  = LinearElastic(E = 210e9, ν = 0.3)
+therm = HeatConductivity(k = 50.2)
+β     = 0.0
+kernel = ThermoElasticKernel(ContinuumFormulation{FullThreeD}(), mech, therm, β)
+
+S = @DOFSet{u::DOF{Displacement{3}, Vertex},
+            T::DOF{Temperature, Vertex}}
+elements, handler = create_elements!(mesh, Element{Hex8, Lagrange{1}, S})
+
+asm   = DOFBasedCOOAssembler()
+cache = DOFBasedCOOCache(elements, handler, mesh, kernel)
+assemble!(cache, asm, kernel, mesh)
+K, f = extract_system(cache)
+
+@assert size(K, 1) == size(K, 2) == 108
+@assert nnz(K) == 5488
+```
+
+## Where to read next
+
+- Kernel source and coupling equations: `src/domains/thermo_elastic/kernel.jl`.
+- Compile-time DOF layout: `local_dof_layout(Element{Hex8, Lagrange{1}, S})`.
+- Matrix-free parity with `K * x`: same test file as above.