Tracer bullet

A thin slice that goes all the way through the system rather than a horizontal layer that goes nowhere alone. Hunt and Thomas's term, and explicitly not a prototype: prototyping generates disposable code, while tracer code is lean but complete and forms part of the final skeleton. Three tests: verifiable alone, PR-sized, ordered by what it unblocks. “Build the data layer” fails all three, because its acceptance criteria are always someone else's.