Executable fused multiply-add dispatch

The dispatcher adds the fixed-limb pair-layout route to the smaller word-specialized kernels without importing their refinement proofs. The pair route contains a complete finite candidate chain; the dispatcher handles only exceptional inputs after that chain returns none.

@[inline]

def FloatLib.Floats.Formats.BinaryInterchange.Model.FmaBackend.dispatch {fmt : FloatFormat} (x y z : Model fmt) : Model fmt

Use the fixed-limb FMA when eligible, otherwise use the word dispatcher.