docs(materials): expand trait docs and align SPDX header

Replace the legacy banner with SPDX lines and document how each
`MaterialBehavior` maps to assembly and `GlobalMaterialCache`, dropping the
unused `AbstractMaterialState` / `state_type` surface in favor of
`material_state_type` NamedTuples.

- Switch the file banner to `SPDX-FileCopyrightText` /
  `SPDX-License-Identifier: MIT`.
- Remove `AbstractMaterialState`, `EmptyState`, and `state_type`; point readers
  at `material_state_type` / `required_state_variables`.
- Enlarge `StatelessConstantTangent`, `StatelessStrainDependent`, and
  `StatefulStrainDependent` docstrings with representative models, tangent
  caveats, and explicit coverage gaps.

src/materials/api.jl

@@ -1,5 +1,5 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
 
 """
 Material model API definitions.
@@ -35,24 +35,10 @@ Stateful plastic materials (history-dependent).
 """
 abstract type AbstractPlasticMaterial <: AbstractMaterial end
 
-"""
-    AbstractMaterialState
-
-Abstract type for material internal state at integration points.
-
-Concrete types must be immutable for thread safety.
-"""
-abstract type AbstractMaterialState end
-
-"""
-    EmptyState <: AbstractMaterialState
-
-Empty material state for stateless materials.
-"""
-struct EmptyState <: AbstractMaterialState end
-
-# Zero constructor for Base.zero compatibility
-Base.zero(::Type{EmptyState}) = EmptyState()
+# Per-integration-point material state is represented as a `NamedTuple` whose
+# field types are derived from `required_state_variables(material)`. The
+# concrete `NamedTuple` type for a given material is produced by
+# `material_state_type` (see `src/materials/global_material_cache.jl`).
 
 # ============================================================================
 # MATERIAL BEHAVIOR TRAITS
@@ -68,21 +54,65 @@ abstract type MaterialBehavior end
 """
     StatelessConstantTangent <: MaterialBehavior
 
-Material with constant tangent modulus (independent of strain).
+Material whose tangent is **constant** with respect to strain at fixed temperature (etc.);
+history plays no role and each integration point can reuse one cached `(σ, 𝔻)` pair on the
+assembly hot path (see [`update_material_cache!`](@ref)).
+
+Representative models: [`LinearElastic`](@ref), [`OrthotropicLinearElastic`](@ref),
+[`HeatConductivity`](@ref), [`HydraulicConductivity`](@ref), [`MoistureDiffusivity`](@ref),
+[`ElementWiseScalarDiffusion`](@ref).
+
+Nonlinear diffusion coefficients still dispatch here only when implemented as a **piecewise
+constant per element** cache (`ElementWiseScalarDiffusion`); strongly nonlinear κ(T) laws would
+need either explicit differentiation or a strain-/temperature-dependent trait branch that does
+not exist yet.
 """
 struct StatelessConstantTangent <: MaterialBehavior end
 
 """
     StatelessStrainDependent <: MaterialBehavior
 
-Material with strain-dependent tangent modulus (no internal state).
+**Nonlinear elasticity or hyperelasticity**: tangent `𝔻` varies with strain (or deformation),
+but there are **no persistent IP history variables** carried through [`GlobalMaterialCache`](@ref).
+
+In continuum assembly, [`update_material_cache!`](@ref) always builds Green–Lagrange strain from
+`F` at each integration point (even if the stress–strain map is written as a small-strain law).
+
+Representative models: [`NeoHookean`](@ref), [`MooneyRivlin`](@ref), [`Yeoh3`](@ref), [`Gent`](@ref).
+
+Ogden, Arruda–Boyce, etc., would also belong here once implemented.
 """
 struct StatelessStrainDependent <: MaterialBehavior end
 
 """
     StatefulStrainDependent <: MaterialBehavior
 
-Material with strain-dependent tangent and internal state variables.
+**Incremental / path-dependent models**: stress and tangent depend on strain **and** on stored
+state (`PlasticStrain`, `DamageVariable`, creep strain, eigenstrain, …). [`needs_state`](@ref)
+is true; [`update_material_cache!`](@ref) reads [`get_old_state`](@ref) and writes back via
+[`set_state!`](@ref).
+
+Tangent semantics vary by model:
+
+- [`PerfectPlasticity`](@ref), [`J2LinearIsotropicPlasticity`](@ref), [`StVenantKirchhoffJ2Plasticity`](@ref),
+  [`ChabocheJ2Plasticity`](@ref): algorithmic elastoplastic tangents of the same *rank-one shear*
+  class used in many textbooks (they are **not** the full spatial tangent unless augmented with
+  the usual deviatoric projection terms).
+- [`ScalarDamageLinearElastic`](@ref): explicit damage stagger; `𝔻` drops derivatives `∂d/∂ε`.
+- [`NortonCreepElastic`](@ref): creep increment is explicit in `Δt`; returned `𝔻` is the elastic
+  modulus only.
+- [`LinearElasticWithEigenstrain`](@ref): `𝔻` is elastic; eigenstrain is updated outside
+  `compute_stress`.
+
+Representative solids: [`PerfectPlasticity`](@ref) (kinematic linear hardening),
+[`J2LinearIsotropicPlasticity`](@ref) (linear isotropic expansion of the yield surface),
+[`StVenantKirchhoffJ2Plasticity`](@ref), [`ScalarDamageLinearElastic`](@ref),
+[`ChabocheJ2Plasticity`](@ref), [`NortonCreepElastic`](@ref),
+[`LinearElasticWithEigenstrain`](@ref).
+
+Still missing for broad classical coverage: pressure-dependent yield (Drucker–Prager,
+Mohr–Coulomb), porous metal (Gurson), nonlinear hardening laws (Voce / Swift) as separate types,
+rate-dependent Perzyna/overstress models, and extended hyperelastic families (Ogden).
 """
 struct StatefulStrainDependent <: MaterialBehavior end
 
@@ -107,13 +137,6 @@ Query whether material has internal state variables.
 """
 needs_state(mat::AbstractMaterial) = material_behavior(mat) isa StatefulStrainDependent
 
-"""
-    state_type(::Type{<:AbstractMaterial}) -> Type{<:AbstractMaterialState}
-
-Return the concrete state type for a given material type.
-"""
-state_type(::Type{<:AbstractMaterial}) = EmptyState
-
 # ============================================================================
 # MATERIAL MODEL FUNCTIONS
 # ============================================================================