Add updateMyVisaNationality mutation: finds the caller's AttendingMember where the program has visaStatusVisibleToMembers=true and a VisaApplication exists, updates VisaApplication.nationality, and emits a VISA_NATIONALITY_SELF_SET audit log. Throws NOT_FOUND when no eligible application exists. Tests: persists update; rejects caller without a visible visa app. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>