17 events are MAP_ITEM or LIST_ITEM: they arrive once per element, all
carrying the same argument names, so a flat store kept only whichever landed
last. SensorsStatesListChanged reports six sensors that way, and the
simulator's deliberate magnetometer fault was invisible through state() as a
result, which is exactly the fact preflight_check needs to see.
Entries are now keyed Command[element]_arg. Parrot marks these events but
never names the key; it is the first argument by convention.
