UCS FACE had no test at all: it was only known to compile. The plane
it builds has two halves and they sit either side of a visibility
boundary, so the coverage splits to match.
tests/ucs_face_plane.rs takes the public route — extrude a disc, then
check the caps come back as distinct planar faces with opposite
outward normals, that an off-centre pick lands on the same face as a
central one, and that the curved wall is never offered as a plane.
The axis construction is pub(crate), so it is unit-tested next to
ucs_from_normal instead: Z is the normal it was handed, the axes stay
orthonormal, a normal along world Z still yields a plane rather than
crossing with itself, the same normal is reproducible, and a zero
vector is refused rather than silently producing a degenerate UCS.
