operators/helpers.cuh already had bounce_back_swap (the uniform steps use it). It moves verbatim, #pragma unroll included, into its own operators/bounce_back_swap.cuh, which helpers.cuh includes; the channel root includes that header and calls it. bounce_back_swap_d2q9 from the previous commit is removed. bounce_back.cuh keeps only the half-way rows helper. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>