feat(legacy): add Poi1/Lagrange element constructors

Provide legacy `Element` factories with Dict-backed geometry fields.

- Implement `elements_lagrange.jl` hooks expected by Abaqus glue and Problems.

src/legacy/elements_lagrange.jl

@@ -0,0 +1,58 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/FEMBase.jl/blob/master/LICENSE
+
+# Poi1: 0D point element (single node, no parametric coordinates)
+struct Poi1 <: AbstractBasis end
+
+# nnodes for Poi1
+nnodes(::Type{Poi1}) = 1
+nnodes(::Poi1) = 1
+
+function get_basis(::E, ::Any, ::Any) where E<:AbstractElement{M,Poi1} where M
+    return [1]
+end
+
+function get_dbasis(::E, ::Any, ::Any) where E<:AbstractElement{M,Poi1} where M
+    return [0]
+end
+
+function (::Element{M,Poi1})(::Any, ::Float64, ::Type{Val{:detJ}}) where M
+    return 1.0
+end
+
+function get_integration_order(::Poi1)
+    return 1
+end
+
+function get_integration_points(::Poi1, ::Int)
+    return [(1.0, (0.0,))]
+end
+
+function size(::Type{Poi1})
+    return (0, 1)
+end
+
+function length(::Type{Poi1})
+    return 1
+end
+
+function get_reference_element_coordinates(::Type{Poi1})
+    Vector{Float64}[[0.0]]
+end
+
+function inside(::Union{Type{Seg2},Type{Seg3},Type{Quad4},
+        Type{Quad8},Type{Quad9},Type{Pyr5},
+        Type{Hex8},Type{Hex20},
+        Type{Hex27}}, xi)
+    return all(-1.0 .<= xi .<= 1.0)
+end
+
+function inside(::Union{Type{Tri3},Type{Tri6},Type{Tri7},
+        Type{Tet4},Type{Tet10}}, xi)
+    return all(xi .>= 0.0) && (sum(xi) <= 1.0)
+end
+
+function get_reference_coordinates(::E) where E<:AbstractElement{M,B} where {M,B}
+    return get_reference_element_coordinates(B)
+end
+