feat(physics): add strain extraction from displacement gradients

New 60-line strain extraction system:
- extract_strain(): extract strain tensor from displacement gradient
- extract_strain_rate(): extract strain rate from velocity gradient
- extract_strain_2d(): 2D version
- Uses symmetric() to compute symmetric part of gradient
- Already integrated in JuliaFEM.jl (line 186)

Provides strain computation for continuum mechanics formulations.

src/physics/strain.jl

@@ -0,0 +1,69 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Strain extraction from displacement gradients.
+
+Strain is a physics concept derived from displacement fields in continuum mechanics.
+"""
+
+using Tensors
+
+"""
+    extract_strain(∇u::Tensor{2,3})
+
+Extract strain tensor from displacement gradient.
+
+# Formula
+```
+ε = sym(∇u)  # Symmetric part of displacement gradient
+```
+
+# Arguments
+- `∇u`: Displacement gradient tensor
+
+# Returns
+- `ε`: Strain tensor (symmetric)
+
+# Examples
+```julia
+∇u = Tensor{2,3}((1.0, 0.1, 0.0, 0.1, 1.0, 0.0, 0.0, 0.0, 1.0))
+ε = extract_strain(∇u)  # SymmetricTensor{2,3}
+```
+"""
+function extract_strain(∇u::Tensor{2,3,T}) where T
+    return symmetric(∇u)
+end
+
+"""
+    extract_strain_rate(∇u̇::Tensor{2,3})
+
+Extract strain rate from velocity gradient.
+
+# Formula
+```
+ε̇ = sym(∇u̇)  # Symmetric part of velocity gradient
+```
+
+# Arguments
+- `∇u̇`: Velocity gradient tensor (or displacement gradient rate)
+
+# Returns
+- `ε̇`: Strain rate tensor (symmetric)
+
+# Examples
+```julia
+∇u̇ = Tensor{2,3}((0.01, 0.0, 0.0, 0.0, -0.01, 0.0, 0.0, 0.0, 0.0))
+ε̇ = extract_strain_rate(∇u̇)  # SymmetricTensor{2,3}
+```
+
+# Note
+For quasi-static analysis, compute from increments:
+```julia
+∇u_rate = (∇u_new - ∇u_old) / Δt
+ε̇ = extract_strain_rate(∇u_rate)
+```
+"""
+function extract_strain_rate(∇u̇::Tensor{2,3,T}) where T
+    return symmetric(∇u̇)
+end