Why are exec and run so confusing?

While working on rv, there’s a specific question that has come up over and over again, in many different forms. In the simplest possible form, it boils down to: What is the difference between rv exec and rv run? Why have both? We haven’t finished implementing either rv exec or rv run yet, but every time one or the other comes up in a conversation, everything instantly becomes more confusing. This post will summarize the history of exec and run in Bundler, npm, Cargo, and uv. Once we have the history laid out, we can take a look at what we plan to do in rv, and you can give us your feedback.