docs(test/materials): document materials test layout

test/materials/README.md

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+# test/materials/
+
+Tests for the material trait system and the material/assembly cache
+machinery used by the DOF-based assembler.
+
+## What is here
+
+State-variable and trait infrastructure:
+
+- `test_state_variables.jl`
+- `test_traits.jl`
+
+Material cache and assembly workspace, including the zero-allocation
+contract that the cantilever bench locks in:
+
+- `test_global_material_cache.jl`
+- `test_assembly_workspace_refactor.jl`
+- `test_cantilever_material_cache_zero_allocations.jl`
+
+End-to-end material-driven assembly:
+
+- `test_plasticity_integration.jl`
+
+All files in this directory are wired into `test/runtests.jl` and run
+on every `Pkg.test()`.
+
+## Related
+
+- Source: `src/materials/`.
+- Concrete model unit tests (`test_linear_elastic`, `test_neo_hookean`,
+  `test_perfect_plasticity`, `test_finite_strain_plasticity`) and the
+  `_new_api.jl` constitutive specifications were moved to
+  `llm/design/legacy-tests/materials/` because they target removed APIs.
+  The materials roadmap lives at `llm/design/materials-tdd/`.