refactor(backend): Remove Vec{3} conversion in CPU backend

- Changed 'ξ = Vec{3}(ip.ξ)' to 'ξ = ip.ξ' in compute_element_stiffness
- No conversion needed since ip.ξ is now already Vec{3}

src/backend/cpu.jl

@@ -142,7 +142,7 @@ function compute_element_stiffness(element::Element{N,NIP,F,B}, time::Float64) w
 
     # Integrate over element
     for ip in ips
-        ξ = Vec{3}(ip.ξ)
+        ξ = ip.ξ
         w = ip.weight
 
         # NEW API: Basis function derivatives (shape function gradients in reference coords)