Driving Issie from outside it, for development and for automated checking.

 Published as `window.issieDev` in a debug build, and reached from a terminal through
 `scripts/drive.js`. It exists because the alternative - synthesising DOM events and reading
 rendered text back - is both slow and wrong often enough to mislead: a click has to find the
 right element, a wait has to guess how long a render takes, and text scraped from the DOM can
 be a frame out of date while looking authoritative.

 Four things, which is what that experience says are needed:

   onNextRender   when the update has been applied AND the view has run, so a caller can wait
                  for the app rather than for a stopwatch
   send           a named command, dispatched as the message the UI would have sent
   state          what the app currently is, as data rather than as rendered text
   simRefs        what is holding a simulation, which is the question the DOM cannot answer

 `send` takes a NAME from a fixed table rather than a serialised Msg. A Msg is an F# union
 carrying models, canvases and functions; nothing useful survives a round trip through JSON, and
 a general dispatch-anything surface in a debug build is a hazard for the sake of messages
 nobody wanted to send. Add a row to `commands` when a new one is needed - that is the point.