KCL: Track Z axis of planes

KCL should track which way a plane is facing. If it only tracks a plane's
X and Y axes, it could identify two separate planes (overlapping
perfectly in space, but one plane's "up" is the other's down).

This is helpful for knowing which way to extrude on a given plane, or
which way to cut "into" a given solid.

docs/kcl-std/types/std-types-Plane.md

@@ -26,6 +26,8 @@ myXY = {
 ```
 
 Any object with appropriate `origin`, `xAxis`, and `yAxis` fields can be used as a plane.
+The plane's Z axis (i.e. which way is "up") will be the cross product X x Y. In other words,
+KCL planes follow the right-hand rule.