This is the lookup the sheet performs per cell (ConditionalFormat._prepareIconSet), lifted to
the whole rule so an editor previews exactly what the grid paints: reverse turns the band
before the override is read, and an override naming a set the table does not hold, or a slot
past its end (Excel's "No Cell Icon"), is null — a band that draws nothing.
The result is indexed by threshold row (stops[i]), so it lines up with the editor's rows
as authored; reverse it for the way a gallery shows a set (highest first).
The icon each band of a rule draws — the set's own, or the band's override — lowest band first.