feat(fields): add LocalField structure for material evaluation

New 62-line LocalField structure:
- LocalField{T,G,R,GR}: field quantities at a point (value, gradient, rate, gradient_rate)
- Unified dynamic and quasi-static treatment (quasi-static: rate=0)
- Supports field values, spatial gradients, time derivatives, gradient rates
- Used for material evaluation at integration points
- Already integrated in JuliaFEM.jl (line 141)

Provides comprehensive field data structure for material constitutive evaluation.

src/fields/local_field.jl

@@ -0,0 +1,62 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Local field evaluation structure for material evaluation.
+
+Provides comprehensive field data at a point including:
+- Field values (T, u, p, etc.)
+- Spatial gradients (∇T, ∇u, ∇p, etc.)
+- Time derivatives (Ṫ, u̇, ṗ, etc.)
+- Time derivatives of gradients (∇Ṫ, ∇u̇, ∇ṗ, etc.)
+"""
+
+using Tensors
+
+"""
+    LocalField{T, G, R, GR}
+
+Field quantities evaluated at a single point (typically an integration point).
+
+# Fields
+- `value::T`: Field value (e.g., displacement `u`, temperature `T`)
+- `gradient::G`: Spatial gradient (e.g., `∇u`, `∇T`)
+- `rate::R`: Time derivative (e.g., velocity `u̇`, temperature rate `Ṫ`)
+- `gradient_rate::GR`: Time derivative of gradient (e.g., `∇u̇`, `∇Ṫ`)
+
+# Unified Dynamic and Quasi-Static Treatment
+
+Quasi-static is treated as a special case of dynamic where `rate = 0`.
+The `gradient_rate` is always computed from increments: `(∇u_new - ∇u_old)/Δt`
+
+**Dynamic simulations:**
+- `rate` = actual time derivative (velocity `u̇`, temperature rate `Ṫ`)
+- `gradient_rate` = computed from increment for accuracy
+
+**Quasi-static simulations:**
+- `rate` = `0` (no dynamic effects)
+- `gradient_rate` = computed from increment (needed for rate-dependent materials)
+
+# Examples
+
+```julia
+# Dynamic case
+u_local = LocalField(u_new, ∇u_new, u̇, ∇u_rate)
+
+# Quasi-static case (rate = 0, but gradient_rate still computed)
+u_local = LocalField(u_new, ∇u_new, zero(Vec{3}), ∇u_rate)
+
+# Temperature field
+T_local = LocalField(T, ∇T, Ṫ, ∇Ṫ)
+```
+"""
+struct LocalField{T, G, R, GR}
+    value::T           # Field value
+    gradient::G        # Spatial gradient
+    rate::R            # Time derivative
+    gradient_rate::GR  # Time derivative of gradient
+end
+
+# Note: create_local_field_namedtuple() was removed because interpolate_local_fields()
+# in src/elements/interpolate.jl directly creates LocalField objects from elements.
+# The helper function was not needed for the actual implementation.