Row index in rows.
Cell index in rows[row].cells (not a grid column).
Columns the cell spans (clamped to ≥ 1).
Rows the cell spans (clamped to ≥ 1).
deg is CCW; 0/90/180/270 snap to exact rotations, else an arbitrary angle.
color: null clears the cell's shading; a triple sets it.
A single editing operation against a GigaDocument model, mirroring the JSON shape of
model::edit::ModelOp. Pass an array to GigaPdfEngine.applyModelOps; ops run in order and out-of-range addresses are no-ops.