A client could only learn the top-level methods from docs/control-protocol.md;
help, methods, ping and the like all came back as "unknown method". The app
and the headless backend now answer control.methods with each method they
serve ({method, params, description}) and ping with "pong", and an unknown
method's error points to control.methods. A test keeps the app's list equal
to the doc table and checks every listed method is dispatched.
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