Function gpu_element_stiffness 

pub fn gpu_element_stiffness(mesh: &GpuFemMesh, e: usize) -> [f64; 4]

Compute the 2×2 element stiffness matrix for bar/rod element e.

For a 1-D bar element: k_e = (E·A / L) · [[1, -1\], [-1, 1]]

Returns [k00, k01, k10, k11] in row-major order.