The port editor gets 'From variable' next to '+ Field': pick a struct variable
of the behavior and its fields land as ports. It only adds - a field the node
already declares stays as it is, because it may carry an edge - and the entry
is disabled once there is nothing left to add. A list of structs counts too:
it declares the shape of every item, which is exactly the bundle a Pack builds
for it. A color field arrives as a number port; the graph has no color type.
AddPortsCommand is new underneath: adding five ports had to be one undo step,
and it checks every key before it appends a single one, so a collision does
not leave half the list behind. AddPortCommand is a three-line subclass of it,
which leaves its sixteen call sites untouched.