Function gpu_heat_diffusion 

pub fn gpu_heat_diffusion(solver: &mut GpuThermalSolver, dt: f64)

Perform one parallel heat-equation update step (mock GPU dispatch).

Applies the explicit finite-difference stencil:

  T_new[i] = T[i] + dt * α * ∇²T[i]

Interior cells only; boundary cells are left unchanged.

Arguments

• solver - The thermal solver (updated in place).
• dt - Time step size (s).