MovePathIfc

The geometry a journey is made through: where the vehicle is, how far things are, and how to say where between two places is.

The half of movement that differs between substrates. InterpolatedMovement is the other half -- the clockwork -- and it is written once because everything substrate-specific is here.

positionAlong may answer null, and that is a supported answer rather than a failure. A plane can say where a third of the way from A to B is; a table of pairwise distances cannot, and neither can a great-circle model without spherical interpolation nobody has asked for. A geometry that cannot say makes a journey one step: the vehicle waits out the whole leg and arrives, which is exactly what a free-path move has always done.