GC designations mapped to one NXT fund and separate custom classification

For GiveCampus integrations and API.

The NXT gift exporter matches each GiveCampus designation backend identifier to a NXT fund lookup identifier. Default Fund fills undesignated amounts; it does not override mapped designations into a single fund. Multiple GC designations can be configured with the verified common fund identifier when that is the intended accounting mapping, but this does not also copy their labels into arbitrary NXT gift custom fields. Review a custom transformation/manual mapping workflow for that extra classification, preserve the designation information in exports, and test before changing school-level codes used by other integrations.