cellOwners

Occupancy grid: cellOwners[rowIndex][columnIndex] is a stable id of the cell that paints that grid slot. Slots covered by the same spanned cell share an id; every other slot (including empty fillers) gets its own unique id so the renderer still strokes separators around them.

The renderer derives per-segment separator lines from this grid so an inner border never crosses a merged (col/row-spanned) region. Empty by default for backward compatibility; always populated by the layout engine.