Use ringPlaneQuaternion (also exported from here), which
knows which WAY the equator tilts (CelestialBody.pole). Applying this as
a rotation about the render X axis — what BodyRings used to do — tilts
the rings the opposite way to the orbit convention (the orbital→render
axis swap is a mirror), drawing a moon at the planet's own tilt twice
that tilt off the ring plane. Kept for existing callers; removed in the
next major.
The ring plane's tilt from the reference plane, radians (see CelestialBody.axialTiltDeg) — magnitude only.