getPointInArcT

Where the angle P falls along the arc from A to B, as a t in [0, 1] for a point on the arc and outside it for a point that is not.

SEMANTICS-ASSUMED: the docs pin 0 at the start and 1 at the end but say nothing about points off the arc. Letting t run on past 1 (rather than clamping) keeps it monotonic all the way round, which is what a caller projecting a pointer onto an arc needs in order to tell which end it is past.