New DX Cluster setting (Settings → DX Cluster). Off (today's behaviour) a call
worked anywhere reads as already worked; on, the WORKED-call flag needs the same
band AND mode. It folds through the digital-mode grouping (Settings → General):
grouped, a 20m FT8 contact also marks a 20m FT4 spot worked; ungrouped they are
separate slots. Backend: qso.WorkedCallSlotKeys builds CALL|band and
CALL|band|mode(grouped) keys; ClusterSpotStatuses uses them when the option is on
(a spot with no inferable mode falls back to same-band).