feat(materials): add continuum kinematics trait tags

Introduce `SmallStrainKinematics` vs `GreenLagrangeKinematics` hooks so
`update_material_cache!` can pick the strain measure per material.

- Define `AbstractContinuumKinematics`, concrete tags, and default
  `continuum_kinematics(::AbstractMaterial)` dispatch.

src/materials/continuum_kinematics.jl

@@ -0,0 +1,31 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+"""
+Continuum kinematics tag for stateful solid materials.
+
+[`update_material_cache!`](@ref) uses [`continuum_kinematics`](@ref) to decide whether
+integration-point strain is the symmetric displacement gradient (`SmallStrainKinematics`)
+or Green–Lagrange strain from `F` (`GreenLagrangeKinematics`). Hyperelastic models already
+use the Green–Lagrange path via [`StatelessStrainDependent`](@ref).
+
+Finite-strain **multiplicative** elastoplasticity is not implemented here; StVK–J₂ uses
+the Green–Lagrange measure with a St. Venant–Kirchhoff elastic predictor (see
+[`StVenantKirchhoffJ2Plasticity`](@ref)).
+"""
+
+abstract type AbstractContinuumKinematics end
+
+"""Symmetric gradient `ε = sym(∇u)` (small-strain assembly)."""
+struct SmallStrainKinematics <: AbstractContinuumKinematics end
+
+"""`E = ½(FᵀF − I)` with `F = I + ∇u ⊗ …` (total Lagrangian assembly)."""
+struct GreenLagrangeKinematics <: AbstractContinuumKinematics end
+
+"""
+    continuum_kinematics(material::AbstractMaterial)
+
+Return [`SmallStrainKinematics`](@ref) by default; materials such as
+[`StVenantKirchhoffJ2Plasticity`](@ref) override with [`GreenLagrangeKinematics`](@ref).
+"""
+continuum_kinematics(::AbstractMaterial) = SmallStrainKinematics()