type CompositionPlacement
Which arm a composition lands on, and the numbers that arm needs.
One decision in one place, because the two arms meet exactly at
MIN_DISTANCE_RADII, and a composition that names 1.04 radii has to land
somewhere: the orbit arm clamps at 1.5, so without this decision there is no
arm for it. sunset is a stance 0.04 radii up, and saying so is what makes
it a planetarium picture rather than something only a hull can take.
toStar is the unit vector from the body toward its star in the axes the
result is wanted in — the reference plane's, for the orbit arm; the body's
own rotating axes, for the surface arm, since a stance is a place on the
ground.