Fix wrong 4th arg in viscous finite-difference gradient call

The finite-difference gradient call passed dq_prim_dy_qp (y-gradient)
as the 4th argument instead of dq_prim_dz_qp (z-gradient). This meant
the z-derivative was never computed in the FD path, and the y-derivative
was written twice.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_viscous.fpp

@@ -985,7 +985,7 @@ contains
                         call s_compute_fd_gradient(q_prim_qp%vf(i), &
                                                    dq_prim_dx_qp(1)%vf(i), &
                                                    dq_prim_dy_qp(1)%vf(i), &
-                                                   dq_prim_dy_qp(1)%vf(i))
+                                                   dq_prim_dz_qp(1)%vf(i))
                     end do
 
                 end if