FillType
type FillType = typeof FILL_TYPES[number];How a fill projects the seed cells into the destination, mirroring Excel’s fill-handle drag:
copy--- repeat the seed without progression.series--- extend each numeric run as an arithmetic series.linearTrend/growthTrend--- fit a least-squares line / exponential and extrapolate. Only offered when a run has two or more numbers (seedSupportsTrend).default--- likeseries, but a single-cell plain number copies.