two safe steps of its own (`trouble-words`: eleven real backend strings through
`__ss.say` on the device, every output asserted free of Rust; and
`trouble-xfer`: one already-failed transfer row written into the store and
taken away again — nothing fetched, and a terminal row is not in flight, so
nothing reaches the shade). Fifteen more mock steps drive the failure faces
themselves, which are described under **When things go wrong**. A mock step can