Capstone - The Zero-Downtime Migration

The scariest changes in production aren't features: they're migrations: changes to the shape of data that's being written 400 times a second by a system that must not stop. This capstone ships one, end to end, under the module's two laws: the deploy/release split, and rollback as a reflex at every single step: including the steps where naive rollback is impossible, which must be restructured until it isn't.