New 230-line API file defining DOF system foundation:
- AbstractDOF: base type for all field specifications
- DOF{T,E}: abstract type for type-level field specs (never instantiated)
- DOFSet: type alias for multi-field specifications (currently NamedTuple)
- dof_size(): utility for counting DOF components (Float64, Vec, Tensor, etc.)
- @DOFSet macro support for clean multi-field syntax
- Comprehensive documentation with examples
- Type-level, compile-time resolved (no runtime overhead)
Provides foundation for unified multi-field DOF system.