docs(legacy): describe opt-in Legacy module scope

Document what ships behind `JULIAFEM_ENABLE_LEGACY=1` and how includes are ordered.

- Summarize Dict fields, Problems/Solvers, and Abaqus IO relocation.

src/legacy/README.md

@@ -0,0 +1,30 @@
+# `src/legacy/` — optional pre-reset API
+
+Everything here loads only when **`JULIAFEM_ENABLE_LEGACY=1`** is set before
+`using JuliaFEM`. Symbols live under **`JuliaFEM.Legacy`** and are not
+re-exported from `JuliaFEM`.
+
+## What is inside (include order from `Legacy.jl`)
+
+1. **`dcti_dvti_fields.jl`** — Dict-backed field containers (`DCTI`, `DVTI`, …).
+2. **`elements_lagrange.jl`** — legacy `Element(Poi1, …)` style constructors.
+3. **`linear_system.jl`** — sparse system helpers used by old solvers.
+4. **`core_types.jl`** — legacy node / integration-point types.
+5. **`assembly_problems.jl`** — `Problem` / field problem hierarchy.
+6. **`analysis.jl`** — analysis drivers and writers.
+7. **`deprecated_fembase.jl`** — compatibility shims with warnings.
+8. **`problems_dirichlet.jl`** — legacy Dirichlet boundary drivers.
+9. **`solvers.jl`** — solver stack.
+10. **`io/`** — Abaqus keyword register, mesh/model parsers, surface helpers, download shim.
+11. **`deprecations.jl`** — small `assemble!` shims and the `Abaqus` helper module.
+
+## Modern replacements (0.x)
+
+| Legacy concept | Current direction |
+|----------------|-----------------|
+| Dict field bags, `register_fields!` | `@DOFSet`, `DOFHandler`, `create_elements!` |
+| `Problem` / `Analysis` drivers | `AbstractKernel` + assemblers + external solvers |
+| `Assembly` multi-mesh container | One `Mesh` per region or explicit coupling code |
+| Old mesh readers | `read_gmsh_mesh` in active `src/io/` where available; Abaqus/Aster here |
+
+See also package **`docs/src/legacy.md`** and **`juliafem.github.io/docs/migration-and-versioning.md`**.