User enclosure evaluation

An extension supplies rational bounds independently of the endpoint representation. Calls check their argument count, decode the inputs, and round the result outward with the selected backend. The checker reevaluates each call on every subdivision; no sampled values or frozen bounds are used.

def FloatLib.Numerics.Interval.Backend.withExtensions {α : Type u_1} (B : Backend α) (config : Config) (extensions : List Extension) : Backend α

Add registered rational enclosures to an existing endpoint backend.