feat(legacy): add Legacy.jl entry module

Wire optional loading of historical Problems, fields, solvers, and Abaqus IO after
the modern core is defined.

- Gate includes on `JULIAFEM_ENABLE_LEGACY` from `JuliaFEM.jl`.

src/legacy/Legacy.jl

@@ -0,0 +1,107 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+    JuliaFEM.Legacy
+
+Older pre-reset API surface. Loaded only when the environment variable
+`JULIAFEM_ENABLE_LEGACY=1` is set at module-load time. Contains the
+Dict-based field system (`DCTI`, `DVTI`, ...), the `Problem` /
+`Assembly` / `Solver` / `Analysis` hierarchy, the `Element(Poi1, ...)`
+constructor family, the Abaqus mesh-reader stack, and the various
+`problems_*` files.
+
+None of this is needed by the current default **0.x** code (`@DOFSet`,
+`Element{K,P,S,N}`, `DOFHandler`, `DOFBasedCOOAssembler`, the matrix-free
+operators, etc.). It is preserved here so that downstream users who
+still depend on the old API can opt back in via the env variable, and
+so that the contact/mortar reference implementations remain accessible
+during the migration.
+
+# Design notes
+
+The legacy files reference each other with late-bound dispatch
+(`update!`, `interpolate`, `field`, `Problem`, ...). They are therefore
+included as a single block, in dependency order, after the new core
+types are available in the parent `JuliaFEM` module.
+
+To make sub-includes resolve relative to this directory, no path
+prefix is needed; `include(\"foo.jl\")` from inside `Legacy.jl` resolves
+to `src/legacy/foo.jl`.
+"""
+module Legacy
+
+using ..JuliaFEM
+# Names the legacy files reference at top level. `Mesh` and
+# `AbstractMaterial` are deliberately NOT imported because the Abaqus
+# parser defines its own (Dict-based) `Mesh` and `AbstractMaterial`
+# types inside this submodule and would otherwise collide with the
+# JuliaFEM bindings.
+using ..JuliaFEM: AbstractField, AbstractElement, Element, AbstractMesh
+using ..JuliaFEM: AbstractBasis, AbstractTopology
+using ..JuliaFEM: ContinuumFormulation, FullThreeD, AbstractContinuumTheory
+using ..JuliaFEM: AbstractFormulation
+using ..JuliaFEM: dofs_per_node, get_field
+using ..JuliaFEM: Seg2, Seg3, Tri3, Tri6, Tri7, Quad4, Quad8, Quad9
+using ..JuliaFEM: Tet4, Tet10, Pyr5, Wedge6, Wedge15, Hex8, Hex20, Hex27
+# Reuse the parent module's no-op `@timeit` stub instead of redefining it
+# locally; the two used to drift independently.
+using ..JuliaFEM: @timeit
+import Base: getindex, setindex!, length, size, haskey, convert, ==
+
+using SparseArrays
+using LinearAlgebra
+using Logging
+
+# CSC-only sparse helpers used by the legacy linear-system / solver path.
+# Not part of the modern public API; loaded with the rest of the legacy
+# stack so downstream callers that opted in to `JULIAFEM_ENABLE_LEGACY=1`
+# keep working.
+include("sparse_helpers.jl")
+
+# Legacy Dict-based field system: DCTI, DVTI, DCTV, DVTV, CCTI, CVTI, CCTV, CVTV, ...
+include("dcti_dvti_fields.jl")
+
+# Legacy element constructors using AbstractBasis{0} (Poi1) and Dict-based fields.
+include("elements_lagrange.jl")
+
+# LinearSystem (sparse-matrix + lambda + ...) used by `Solver`/`Analysis`.
+include("linear_system.jl")
+
+# Legacy core types (Node, IP, IntegrationPoint, Point).
+include("core_types.jl")
+
+# Legacy Problem hierarchy (FieldProblem / BoundaryProblem / Problem).
+include("assembly_problems.jl")
+
+# Analysis (Analysis, AbstractAnalysis, AbstractResultsWriter).
+include("analysis.jl")
+
+# Deprecated FEMBase methods (length / size / setindex! / update! warnings).
+include("deprecated_fembase.jl")
+
+# Boundary-condition problem (Dirichlet).
+include("problems_dirichlet.jl")
+
+# Solver hierarchy (Solver = Analysis, Linear, Nonlinear).
+include("solvers.jl")
+
+# Mesh I/O stack (Abaqus). `parse_mesh.jl` provides the working
+# `abaqus_read_mesh`; `parse_model.jl` provides `abaqus_read_model`;
+# `create_surface_elements.jl` is shared infrastructure used by the
+# Dict-based mesh. All depend on the legacy `Element(Poi1, ...)`
+# constructor and the Dict-based fields above. Aster `.med` reading
+# requires HDF5 and is intentionally not wired in here.
+#
+# Loaded before `deprecations.jl` so the `Abaqus` sub-module's
+# `import ..create_surface_elements` finds its target.
+include("io/keyword_register.jl")
+include("io/parse_mesh.jl")
+include("io/parse_model.jl")
+include("io/create_surface_elements.jl")
+include("io/abaqus_download.jl")
+
+# Tiny `assemble!` shims plus the `Abaqus` sub-module helper.
+include("deprecations.jl")
+
+end # module Legacy