doctor
Print, engine by engine, whether it is installed and how to install it.
Installed (output verbatim; compiler messages are Chinese-only for now):
Not installed:
A missing engine exits 3 (environment missing), not 1. They are separate so CI can distinguish "the document is wrong" from "the machine is missing software".
One line in this version
Only Mermaid is implemented, so there is only one line. The rest are in The other seven diagram types.
When PlantUML lands, doctor will check java -version and give a clear diagnostic when it is missing, rather than letting a raw spawn failure surface.