feat(dofs): add DOF system main entry point

New 40-line main entry point for DOF system:
- Includes api.jl (DOF{T,E} abstract type)
- Includes fields.jl (field specification system)
- Documents multi-field philosophy (single-field is special case)
- Defines module structure and dependencies
- Already integrated in JuliaFEM.jl (line 313)

Provides unified type-level field specification system for finite elements.

src/dofs/dofs.jl

@@ -0,0 +1,40 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+
+"""
+DOF System - Type-Level Field Specifications
+
+Main entry point for the DOF system.
+
+# Philosophy (November 2025)
+
+**Multi-field is fundamental! Single-field is just a special case with one key.**
+
+Field specifications are `DOFSet` types (currently implemented as NamedTuples), used purely at the type level:
+
+```julia
+# Clean syntax using abstract DOF{T,E} types:
+(T = DOF{Float64, Vertex}, u = DOF{Vec{3,Float64}, Vertex})
+
+# Using DOFSet macro (preferred):
+S = @DOFSet{T::DOF{Temperature, Vertex}, u::DOF{Displacement{3}, Vertex}}
+
+# Used in Element type parameter:
+Element{Tetrahedron{4}, Lagrange{1}, S}
+```
+
+**Note**: `@NamedTuple` also works (since `DOFSet = NamedTuple`), but `@DOFSet` is preferred for future compatibility.
+
+# Module Structure
+- `api.jl`: DOF{T,E} abstract type + dof_size() utility
+- `fields.jl`: Field specification system (@DOFSet, ndofs, etc.)
+"""
+
+# DOF{T,E} abstract type (type-level specification only)
+include("api.jl")
+
+# Field specification system (@DOFSet macro, ndofs, etc.)
+include("fields.jl")
+
+# Note: dof_connectivity.jl is included later in JuliaFEM.jl after Element is defined
+# Note: dof_manager.jl is included later in JuliaFEM.jl after Mesh is defined