pub fn kernel_spiky_grad(r_vec: [f64; 3], r: f64, h: f64) -> [f64; 3]
Gradient of the Spiky kernel: ∇W_spiky(r_vec, r, h).
Returns −45/(π·h⁶) · (h−r)² · r̂ for r < h and r > 0, else zero vector.
−45/(π·h⁶) · (h−r)² · r̂
r < h
r > 0