refactor(materials): Add @inline to PerfectPlasticity functions

- Added @inline to is_finite_strain, has_state
- Trait functions called in assembly loops
- Performance optimization for plastic materials

src/materials/perfect_plasticity.jl

@@ -92,25 +92,25 @@ using LinearAlgebra
 include("abstract_material.jl")
 
 """
-    PlasticityState
+    PlasticityState <: AbstractMaterialState
 
 State variables for perfect plasticity model.
 
 # Fields
-- `ε_p::SymmetricTensor{2,3,Float64}` - Plastic strain tensor
-- `α::SymmetricTensor{2,3,Float64}` - Backstress (kinematic hardening)
+- `ε_p::SymmetricTensor{2,3,Float64,6}` - Plastic strain tensor
+- `α::SymmetricTensor{2,3,Float64,6}` - Backstress (kinematic hardening)
 - `κ::Float64` - Equivalent plastic strain (scalar)
 
 # Notes
 Immutable for thread safety. Updates create new state.
 """
-struct PlasticityState
-    ε_p::SymmetricTensor{2,3,Float64}  # Plastic strain
-    α::SymmetricTensor{2,3,Float64}    # Backstress
-    κ::Float64                          # Equivalent plastic strain
+struct PlasticityState <: AbstractMaterialState
+    ε_p::SymmetricTensor{2,3,Float64,6}  # Plastic strain
+    α::SymmetricTensor{2,3,Float64,6}    # Backstress
+    κ::Float64                            # Equivalent plastic strain
 
-    function PlasticityState(ε_p::SymmetricTensor{2,3,Float64},
-        α::SymmetricTensor{2,3,Float64},
+    function PlasticityState(ε_p::SymmetricTensor{2,3,Float64,6},
+        α::SymmetricTensor{2,3,Float64,6},
         κ::Float64)
         κ ≥ 0.0 || throw(ArgumentError("Equivalent plastic strain must be non-negative, got κ = $κ"))
         new(ε_p, α, κ)
@@ -126,6 +126,9 @@ PlasticityState() = PlasticityState(zero(SymmetricTensor{2,3}),
     zero(SymmetricTensor{2,3}),
     0.0)
 
+# Zero constructor for Base.zero compatibility
+Base.zero(::Type{PlasticityState}) = PlasticityState()
+
 """
     PerfectPlasticity <: AbstractPlasticMaterial
 
@@ -214,6 +217,9 @@ PerfectPlasticity(; E::Real, ν::Real, σ_y::Real, H::Real) =
 # Trait declaration: PerfectPlasticity has strain-dependent tangent and state
 material_behavior(::PerfectPlasticity) = StatefulStrainDependent()
 
+# State type trait: PerfectPlasticity uses PlasticityState
+state_type(::Type{PerfectPlasticity}) = PlasticityState
+
 """
     compute_stress(material::PerfectPlasticity, 
                    ε::SymmetricTensor{2,3},