Fix y-velocity perturbation using already-modified x-velocity

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

Author: sbryngelson

src/pre_process/m_perturbation.fpp

@@ -81,8 +81,8 @@ contains
                     perturb_alpha = q_prim_vf(E_idx + perturb_flow_fluid)%sf(i, j, k)
                     call random_number(rand_real)
                     rand_real = rand_real*perturb_flow_mag
-                    q_prim_vf(mom_idx%beg)%sf(i, j, k) = (1._wp + rand_real)*q_prim_vf(mom_idx%beg)%sf(i, j, k)
                     q_prim_vf(mom_idx%end)%sf(i, j, k) = rand_real*q_prim_vf(mom_idx%beg)%sf(i, j, k)
+                    q_prim_vf(mom_idx%beg)%sf(i, j, k) = (1._wp + rand_real)*q_prim_vf(mom_idx%beg)%sf(i, j, k)
                     if (bubbles_euler) then
                         q_prim_vf(alf_idx)%sf(i, j, k) = (1._wp + rand_real)*q_prim_vf(alf_idx)%sf(i, j, k)
                     end if