feat(dofs): add mixed DOF system for coupled fields

New 95-line file for mixed/coupled DOF systems:
- MixedDOF{DOFs}: composition of multiple entity-based DOF types
- Helper functions: is_mixed(), get_dof_types(), num_fields(), get_field_dof_type()
- Support for Stokes (velocity+pressure), 3-field formulations, Taylor-Hood
- DOF ordering: concatenated in field order
- Display formatting for mixed DOF types

Enables mixed interpolation methods with different DOF types per field.

src/dofs/mixed.jl

@@ -0,0 +1,95 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+
+"""
+Mixed DOF Types - Entity-Based
+
+Implementation of mixed/coupled DOF systems where multiple DOF types
+coexist on the same element.
+
+# Core Type
+
+- `MixedDOF{DOFs}`: Composition of multiple entity-based DOF types
+
+# Use Cases
+
+**Stokes (velocity + pressure):**
+```julia
+MixedDOF{(
+    DOF{Vec{2}, Vertex{Triangle{3}}},    # Velocity at vertices
+    DOF{Float64, Vertex{Triangle{3}}}    # Pressure at vertices
+)}
+```
+
+**3-Field Formulation (u + p + J):**
+```julia
+MixedDOF{(
+    DOF{Vec{3}, Vertex{Tetrahedron{4}}},    # Displacement at vertices
+    DOF{Float64, Vertex{Tetrahedron{4}}},   # Pressure at vertices
+    DOF{Float64, Edge{Tetrahedron{4}}}      # Volumetric DOF on edges
+)}
+```
+
+**Mixed Interpolation (Taylor-Hood):**
+```julia
+MixedDOF{(
+    DOF{Vec{2}, Vertex{Triangle{6}}},    # Quadratic velocity
+    DOF{Float64, Vertex{Triangle{3}}}    # Linear pressure (different topology!)
+)}
+```
+
+# DOF Ordering
+
+Mixed DOFs are concatenated in order:
+```julia
+MixedDOF{(DOF{Vec{2}, Vertex{Triangle{3}}}, DOF{Float64, Vertex{Triangle{3}}})}
+# For Triangle{3}:
+# Field 1: [u1x, u1y, u2x, u2y, u3x, u3y]  (6 DOFs from 2×3 vertices)
+# Field 2: [p1, p2, p3]                     (3 DOFs from 1×3 vertices)
+# Total:   [u1x, u1y, u2x, u2y, u3x, u3y, p1, p2, p3]  (9 DOFs)
+```
+
+"""
+
+# MixedDOF is already defined in api.jl, this file adds helper functions
+
+# ============================================================================
+# Helper Functions
+# ============================================================================
+
+"""
+Check if DOF type is mixed.
+"""
+is_mixed(::Type{<:MixedDOF}) = true
+is_mixed(::Type{<:DOF}) = false
+
+"""
+Get the tuple of constituent DOF types.
+"""
+function get_dof_types(::Type{MixedDOF{DOFs}}) where {DOFs}
+    return DOFs.parameters
+end
+
+"""
+Get number of fields in mixed system.
+"""
+function num_fields(::Type{MixedDOF{DOFs}}) where {DOFs}
+    return length(DOFs.parameters)
+end
+
+"""
+Get DOF type for specific field (1-indexed).
+"""
+function get_field_dof_type(::Type{MixedDOF{DOFs}}, field_idx::Int) where {DOFs}
+    return DOFs.parameters[field_idx]
+end
+
+# ============================================================================
+# Display
+# ============================================================================
+
+function Base.show(io::IO, ::Type{MixedDOF{DOFs}}) where {DOFs}
+    dof_types = [string(T) for T in DOFs.parameters]
+    print(io, "MixedDOF{(", join(dof_types, ", "), ")}")
+end
+