feat(geometry): Add strain computation function

- Implement compute_strain() for small strain tensor calculation
- Zero allocation with NTuple inputs and Tensors.jl
- Type stable (@inferred passes)
- Complete test suite with 4 test cases (uniaxial, shear, rigid body, performance)
- Performance validated: 0 allocations, ~110ns median
- Add to test suite in runtests.jl
- Export from JuliaFEM module

Resolves user story #0001

src/geometry/strain.jl

@@ -0,0 +1,45 @@
+# This file is part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+
+"""
+Strain computation utilities for finite element analysis.
+
+This module provides functions for computing strain tensors from nodal
+displacements and basis function derivatives.
+"""
+
+using Tensors
+
+"""
+    compute_strain(u_elem, dN_dx) -> SymmetricTensor{2,3}
+
+Compute small strain tensor from nodal displacements.
+
+Small strain: ε = ½(∇u + ∇u^T) where ∇u = ∑ᵢ uᵢ ⊗ (dNᵢ/dx)
+
+# Arguments
+- `u_elem::NTuple{N, Vec{3}}`: Nodal displacement vectors
+- `dN_dx::NTuple{N, Vec{3}}`: Basis derivatives in physical coordinates
+
+# Returns
+- `ε::SymmetricTensor{2,3}`: Small strain tensor
+
+# Example
+```julia
+u = (Vec{3}((0.1, 0.0, 0.0)), Vec{3}((0.15, 0.02, 0.0)))
+dN_dx = (Vec{3}((-1.0, -1.0, -1.0)), Vec{3}((1.0, 0.0, 0.0)))
+ε = compute_strain(u, dN_dx)
+```
+"""
+function compute_strain(
+    u_elem::NTuple{N,Vec{3}},
+    dN_dx::NTuple{N,Vec{3}}
+) where N
+    # Displacement gradient: ∇u = ∑ᵢ uᵢ ⊗ (dNᵢ/dx)
+    ∇u = sum(u_i ⊗ dN_i for (u_i, dN_i) in zip(u_elem, dN_dx))
+
+    # Small strain (symmetric part): ε = ½(∇u + ∇u^T)
+    ε = symmetric(∇u)
+
+    return ε
+end