nmra watchdog seems to lose a byte of idle from the beginning somehow
 (don't know if this happens with host-supplied data)


watchdog should disable CDU


residual stuff from CDU is present and changes pt0 when you
  * turn on
  * say `+' to master to turn on CDU
  * switch to programming mode
 pt0 on pic2 is the one nearest the switch box, moving to hacker's right
 (we think - I haven't checked this yet)


layout polarity diagram colourful segment encoding:
 make moveable feature selection in subseg2display bitwise rather than
  setwise

safety system no crashes at crossovers please