BlockStep (slot-reservoir wall rows) and BlockChannelStep computed the global cell of a block-window cell with the same four lines. block_global_xy (core/layout.cuh, 2D, __forceinline__) now holds them; both kernels call it with their own domain width. Same integer operations: no numerical, ABI or scheduling change. The slot-reservoir kernel layout is unchanged (no split). Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>