bynk-driver — the shared front-end of the bynkc and bynk CLIs (#521).
Both binaries expose fmt and check with identical semantics; before
this crate each re-implemented the command bodies (and the project-failure
flattening layer, and the project-rooting rule) as by-hand copies pinned
only by comments and a skip-able parity test. The single implementation
lives here, parameterised by the program name that prefixes messages.