A unit direction from a body's center, expressed in that body's rotating axes.
Branded because a bare Vec3 carries no axes and this is the one place where
passing the wrong ones is both easy and silent: terrain is a function of
position on the turning body, so an inertial direction leaves the mountains
standing still in inertial space while the planet rotates underneath them.
That shipped once as "a ship landing 83 m above the ground it had just
touched", and it shipped a second time in the drag path, where nothing
rendered wrong and nothing failed — the density was simply computed against
the wrong mountain.
AGENTS.md enumerates three producers — bodyFixedDirection,
geodeticDirection and regionDirection — and that list is the enforcement
mechanism, not a summary of it: none of the three can be reached without a
spin frame, a latitude/longitude or a region address, which is what makes the
brand worth its cost. faceToDirection below is the primitive regionDirection
is built from and is branded for that reason; production code goes through
regionDirection, so the list stays three long.