feat(dofs): add entity-based DOF convenience types

New 169-line file with convenience types and helpers:
- ScalarDOF{E}: type alias for scalar DOFs at entities
- VectorDOF{D,E}: type alias for D-dimensional vector DOFs
- TensorDOF{D,E}: type alias for tensor DOFs
- Convenience constructors for common DOF patterns
- Helper functions for entity-based DOF operations

Provides convenient type aliases for common DOF patterns using entity-based system.

src/dofs/entity_dofs.jl

@@ -0,0 +1,168 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+
+"""
+Entity-Based DOF Implementations
+
+This file provides convenient type aliases and helper functions for common
+DOF patterns using the entity-based DOF system.
+
+The core type `DOF{T, E<:TopologicalEntity}` is defined in api.jl.
+This file adds convenience constructors and commonly used patterns.
+"""
+
+# ============================================================================
+# Convenience Type Aliases
+# ============================================================================
+
+"""
+    ScalarDOF{E<:TopologicalEntity}
+
+Type alias for scalar DOFs at entities.
+
+# Examples
+```julia
+ScalarDOF{Vertex}      # Scalar at vertices (any topology)
+ScalarDOF{Edge}        # Scalar on edges (any topology)
+ScalarDOF{Face}        # Scalar on faces (any topology)
+```
+
+Equivalent to `DOF{Vec{1}, E}` (scalar as 1-component vector).
+"""
+const ScalarDOF{E} = DOF{Vec{1}, E} where {E<:TopologicalEntity}
+
+"""
+    VectorDOF{D, E<:TopologicalEntity}
+
+Type alias for D-dimensional vector DOFs at entities.
+
+# Examples
+```julia
+VectorDOF{2, Vertex}   # 2D displacement at vertices
+VectorDOF{3, Vertex}   # 3D displacement at vertices
+VectorDOF{3, Edge}     # 3D field on edges
+```
+
+Equivalent to `DOF{Vec{D}, E}`.
+"""
+const VectorDOF{D, E} = DOF{Vec{D}, E} where {D, E<:TopologicalEntity}
+
+"""
+    TensorDOF{D, E<:TopologicalEntity}
+
+Type alias for D×D tensor DOFs at entities.
+
+# Examples
+```julia
+TensorDOF{2, Vertex}   # 2D stress/strain at vertices
+TensorDOF{3, Vertex}   # 3D stress/strain at vertices
+```
+
+Equivalent to `DOF{Tensor{2,D}, E}`.
+"""
+const TensorDOF{D, E} = DOF{Tensor{2,D,Float64}, E} where {D, E<:TopologicalEntity}
+
+# ============================================================================
+# Common DOF Patterns
+# ============================================================================
+
+# Standard Lagrange element DOF types for common problems
+const LagrangeHeat1D = DOF{Vec{1}, Vertex}
+const LagrangeHeat2D = DOF{Vec{1}, Vertex}
+const LagrangeHeat3D = DOF{Vec{1}, Vertex}
+
+const LagrangeElasticity1D = DOF{Vec{1}, Vertex}
+const LagrangeElasticity2D = DOF{Vec{2}, Vertex}
+const LagrangeElasticity3D = DOF{Vec{3}, Vertex}
+
+# Nédélec (H(curl)) element DOF types for electromagnetic problems
+const NedelecScalar1D = DOF{Vec{1}, Edge}
+const NedelecScalar2D = DOF{Vec{1}, Edge}
+const NedelecScalar3D = DOF{Vec{1}, Edge}
+const NedelecVector3D = DOF{Vec{3}, Edge}
+
+# Raviart-Thomas (H(div)) element DOF types for fluid flow problems
+const RaviartThomasScalar2D = DOF{Vec{1}, Face}
+const RaviartThomasScalar3D = DOF{Vec{1}, Face}
+const RaviartThomasVector3D = DOF{Vec{3}, Face}
+
+# Discontinuous Galerkin (DG) element DOF types
+const DGScalar = DOF{Vec{1}, Cell}
+const DGVector2D = DOF{Vec{2}, Cell}
+const DGVector3D = DOF{Vec{3}, Cell}
+
+# ============================================================================
+# Helper Functions
+# ============================================================================
+
+"""
+    make_vertex_dof(::Type{T}) where T
+
+Create DOF type for quantity T at vertices.
+
+Topology context comes from Element.
+
+# Examples
+```julia
+make_vertex_dof(Vec{1})    # DOF{Vec{1}, Vertex}
+make_vertex_dof(Vec{3})    # DOF{Vec{3}, Vertex}
+```
+"""
+make_vertex_dof(::Type{T}) where T = DOF{T, Vertex}
+
+"""
+    make_edge_dof(::Type{T}) where T
+
+Create DOF type for quantity T on edges.
+
+Topology context comes from Element.
+
+# Examples
+```julia
+make_edge_dof(Vec{1})      # DOF{Vec{1}, Edge}
+make_edge_dof(Vec{3})      # DOF{Vec{3}, Edge}
+```
+"""
+make_edge_dof(::Type{T}) where T = DOF{T, Edge}
+
+"""
+    make_face_dof(::Type{T}) where T
+
+Create DOF type for quantity T on faces.
+
+Topology context comes from Element.
+
+# Examples
+```julia
+make_face_dof(Vec{1})      # DOF{Vec{1}, Face}
+make_face_dof(Vec{3})      # DOF{Vec{3}, Face}
+```
+"""
+make_face_dof(::Type{T}) where T = DOF{T, Face}
+
+"""
+    make_cell_dof(::Type{T}) where T
+
+Create DOF type for quantity T in cell interior.
+
+Topology context comes from Element.
+
+# Examples
+```julia
+make_cell_dof(Vec{1})      # DOF{Vec{1}, Cell}
+make_cell_dof(Vec{3})      # DOF{Vec{3}, Cell}
+```
+"""
+make_cell_dof(::Type{T}) where T = DOF{T, Cell}
+
+# ============================================================================
+# Exports
+# ============================================================================
+
+export ScalarDOF, VectorDOF, TensorDOF,
+       LagrangeHeat1D, LagrangeHeat2D, LagrangeHeat3D,
+       LagrangeElasticity1D, LagrangeElasticity2D, LagrangeElasticity3D,
+       NedelecScalar1D, NedelecScalar2D, NedelecScalar3D, NedelecVector3D,
+       RaviartThomasScalar2D, RaviartThomasScalar3D, RaviartThomasVector3D,
+       DGScalar, DGVector2D, DGVector3D,
+       make_vertex_dof, make_edge_dof, make_face_dof, make_cell_dof