function stanceToward
surfaceStance.ts:211
A forward direction, as the heading and pitch a stance holds.
The inverse of the along/forward construction below, and it lives beside
it because the two have to agree about what heading 0 means: north, swinging
toward east, measured in the triad localTriad builds on the pole of
whatever axes up is expressed in. That last clause is the whole reason
this is a function rather than four lines at each call site — a heading
solved in one frame and applied in another is a silent rotation about the
local vertical, and on a tilted body it is the axial tilt's worth of it.
up and forward must therefore be in the same axes, and the answer is
about those axes.
Returns{ heading: number; pitch: number }