feat(materials): add material field trait system

New 140-line material field trait system:
- required_material_fields(): define fields required by physics (Elasticity, Thermal)
- material_field_type(): infer field NamedTuple type from material's supported physics
- compose_field_types(): compose multiple NamedTuple types for multi-physics
- Supports Elasticity (σ, 𝔻) and Thermal (q, k) physics
- Type-stable field type inference
- Already integrated in JuliaFEM.jl (line 431)

Provides compositional field design for material-physics coupling.

src/materials/field_traits.jl

@@ -0,0 +1,140 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Material field trait functions for compositional field design.
+"""
+
+using Tensors
+using ..JuliaFEM: AbstractPhysics, Elasticity, Thermal
+
+# ============================================================================
+# MATERIAL FIELD TRAITS
+# ============================================================================
+
+"""
+    required_material_fields(physics::AbstractPhysics) -> Type{<:NamedTuple}
+
+Returns NamedTuple type defining material fields required by this physics.
+"""
+function required_material_fields end
+
+# ============================================================================
+# PHYSICS-SPECIFIC IMPLEMENTATIONS
+# ============================================================================
+
+"""
+    required_material_fields(::Elasticity{Dim}) where Dim
+
+Elasticity physics requires stress (σ) and tangent modulus (𝔻).
+"""
+function required_material_fields(::Elasticity{Dim}) where Dim
+    if Dim == 3
+        return NamedTuple{
+            (:σ, :𝔻),
+            Tuple{
+                SymmetricTensor{2,3,Float64,6},
+                SymmetricTensor{4,3,Float64,36}
+            }
+        }
+    elseif Dim == 2
+        return NamedTuple{
+            (:σ, :𝔻),
+            Tuple{
+                SymmetricTensor{2,2,Float64,3},
+                SymmetricTensor{4,2,Float64,9}
+            }
+        }
+    else
+        error("Elasticity{Dim} with Dim=$Dim not supported")
+    end
+end
+
+"""
+    required_material_fields(::Thermal{Dim}) where Dim
+
+Thermal physics requires heat flux (q) and thermal conductivity (k).
+"""
+function required_material_fields(::Thermal{Dim}) where Dim
+    if Dim == 3
+        return NamedTuple{
+            (:q, :k),
+            Tuple{
+                Vec{3,Float64},
+                SymmetricTensor{2,3,Float64,6}
+            }
+        }
+    elseif Dim == 2
+        return NamedTuple{
+            (:q, :k),
+            Tuple{
+                Vec{2,Float64},
+                SymmetricTensor{2,2,Float64,3}
+            }
+        }
+    else
+        error("Thermal{Dim} with Dim=$Dim not supported")
+    end
+end
+
+# ============================================================================
+# FIELD TYPE INFERENCE
+# ============================================================================
+
+"""
+    material_field_type(material::AbstractMaterial) -> Type{<:NamedTuple}
+
+Infer material field NamedTuple type from material's supported physics.
+"""
+function material_field_type(material::AbstractMaterial)
+    physics_tuple = supported_physics(material)
+    
+    if isempty(physics_tuple)
+        # No physics - empty fields
+        return NamedTuple{(),Tuple{}}
+    elseif length(physics_tuple) == 1
+        # Single physics - direct mapping
+        return required_material_fields(physics_tuple[1])
+    else
+        # Multiple physics - compose fields
+        field_types = map(p -> required_material_fields(p), physics_tuple)
+        return compose_field_types(field_types...)
+    end
+end
+
+"""
+    compose_field_types(nt_types::Type{<:NamedTuple}...) -> Type{<:NamedTuple}
+
+Compose multiple NamedTuple types into one.
+"""
+@generated function compose_field_types(nt_types::Type{<:NamedTuple}...)
+    # Collect all field names and types
+    all_names = Symbol[]
+    all_types = Type[]
+    
+    for nt_type in nt_types.parameters
+        names = fieldnames(nt_type)
+        types = [fieldtype(nt_type, name) for name in names]
+        append!(all_names, names)
+        append!(all_types, types)
+    end
+    
+    # Build composed NamedTuple type
+    names_tuple = Tuple(all_names)
+    types_tuple = Tuple(all_types)
+    
+    return :(NamedTuple{$names_tuple, $types_tuple})
+end
+
+"""
+    create_zero_field(::Type{FieldType}) where {FieldType<:NamedTuple}
+
+Create zero-initialized field NamedTuple.
+"""
+@generated function create_zero_field(::Type{FieldType}) where {FieldType<:NamedTuple}
+    names = fieldnames(FieldType)
+    types = [fieldtype(FieldType, name) for name in names]
+    zero_values = [:(zero($T)) for T in types]
+    
+    return :(NamedTuple{$names}(($(zero_values...),)))
+end