[Brightcolours]

Is it really possible to have a "measuring stick" lens for a given FL (24mm in this case), anyway? One that can be declared the one to give a true 24mm FOV?

Lenses designed primarily for still photography almost universally suffer from focus breathing. Convention would dictate that the nominal focal length would be at infinity focus.
It follows that, since most lenses breathe to some degree, exact focal length is rather academic - more so in zoom lenses.

True Cine lenses are a different kettle of fish.

With focal length staying the same, fov "breaths".