plus

Combines this provider with other. When both return non-null DayInfo for a date:

• The entry with the higher DayInfo.priority supplies the primary DayInfo.label, DayInfo.colorArgb, and DayInfo.payload (ties favor this).

• If that entry's DayInfo.label is null, the other entry's label is used, so a label-less holiday entry does not erase e.g. a lunar label.

• DayInfo.isHoliday is this.isHoliday || other.isHoliday.

• DayInfo.tags is the deduplicated union of both tag lists in order.