Columns + CellWidth/CellHeight + a row-major CellOffset/LayoutCells pass.
CellWidth<=0 keeps the single-cell fill mode (toolbar slot sizes to the
list — unchanged); CellWidth>0 tiles cells in a grid. Layout runs on
AddItem and per-frame in OnDraw so a list resize reflows.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
