const PLATE_MARGIN: 0.25
The widest margin any band reads a plate property over, radians.
It is the support radius of the weights below: a plate more than this much
farther from the sample than the nearest one is has no say in anything, so
plateAt need not carry it. Every band's own margin has to be no wider —
geology.test.ts holds that, because a band asking for a blend over a margin
the search already truncated would be dividing by a sum that had lost a term
still carrying weight.