Adds a new `atan(_)` primitive method to the Num class that wraps the C
`atan2` function, allowing callers to compute the arc tangent of the
number divided by a given argument `x` while using the signs of both
numbers to determine the correct quadrant of the result. The
implementation registers the primitive `num_atan2` in the core VM
initialization and documents the new method in the core library
reference.