-detector on detector seems to spuriously set B1 or B2 or at least
- panic DR when we honour it
+nmra watchdog seems to lose a byte of idle from the beginning somehow
+ (don't know if this happens with host-supplied data)
-crash readout: master i2c should go into known state
layout polarity diagram colourful segment encoding:
make moveable feature selection in subseg2display bitwise rather than