Create a relationship between two mnemon entries. All 10 labels: MEMBER (NPC ∈ Faction, bidirectional), ALLY (bidirectional), ENEMY (directional), RIVAL (directional), PARENT_OF (Location hierarchy — sourceEntryId is the outer/larger place, e.g. Region → City → District → Tavern), CONTAINS (Location → NPC present there), LOCATED_IN (NPC → Location; inverse of CONTAINS), HAS_SUBQUEST (Quest → subquest Quest), QUEST_RELATED_NPC (Quest → NPC), QUEST_RELATED_LOCATION (Quest → Location). sourceEntryId is the 'from' side; targetEntryId is the 'to' side — direction matters. Call describe_mnemon_types for the full valid (sourceType, label, targetType) matrix. For faction membership prefer memberNpcEntryIds / affiliationEntryIds on the NPC itself; for quest links prefer subQuestEntryIds / relatedNpcEntryIds / relatedLocationEntryIds on the quest.