docs(test): add top-level test tree README

Describe how topic `runtests.jl` files mirror `src/` and how to filter suites.

- Point contributors at ARGS topic filtering via `test/runtests.jl`.

test/README.md

@@ -0,0 +1,46 @@
+# JuliaFEM.jl package tests
+
+The active tree mirrors `src/`. `test/runtests.jl` includes one topic bundle per
+row in the `TOPICS` array:
+
+- `basis`, `fields`, `physics`, `elements`, `materials`, `topology`
+- `domains/continuum`, `domains/heat`, `domains/thermo_elastic`
+- `dofs`, `assemblers`, `validation`, `reference`
+
+Each topic directory contains its own `runtests.jl` that aggregates `@testset`
+files in that folder.
+
+## Commands
+
+```bash
+julia --project=. -e 'using Pkg; Pkg.test()'
+```
+
+GitHub Actions runs the full suite on Julia 1.10 and latest stable, and a
+parallel **assemblers-only** job (latest stable, no coverage) via
+`test_args` — see `.github/workflows/CI.yml` job `test-assemblers`.
+
+Filter to one or more topics via `test_args` (strings must match `TOPICS`
+in `test/runtests.jl`, e.g. `assemblers`, `dofs`, `domains/heat`):
+
+```bash
+julia --project=. -e 'using Pkg; Pkg.test(; test_args=["assemblers"])'
+```
+
+Same filtering when invoking the runner directly:
+
+```bash
+julia --project=. test/runtests.jl assemblers
+julia --project=. test/runtests.jl assemblers dofs
+```
+
+## Older / Legacy tests
+
+Specs that still target removed APIs or experimental layouts may live under
+`llm/design/legacy-tests/` (often gitignored). They are **not** picked up by
+`test/runtests.jl`.
+
+## Related
+
+- `test/materials/README.md` — materials-specific layout.
+- Website-only notes under `juliafem.github.io/test/` are **not** this suite.