I'm well aware that there's no absolute standards for this, but I've built a number of CQRS systems, and I didn't ever have idempotency IDs as a core tenant of the system.

What literature, or framework or ecosystem holds this to be a hard-and-fast requirement of CQRS?

Personally I've solved this problem similar to Git, modelling CQRS on top of Merkle trees so your system at least knows if your incoming edit (command|commit) is being applied on HEAD (no changes to the model since the command started) or to HEAD+n (needing to re-run some command validations, and "rebase" the incoming command).