ENH: option to check merge is one-to-one, many-to-one, one-to-many, or many-to-many

ENH: option to check merge is one-to-one, many-to-one, one-to-many, or many-to-many: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). In my experience, there are few places in data work where problems with data are more evident than when merging datasets -- something that is…