Executable square-root dispatch

The dispatcher adds the certified pair-layout route to the smaller word-specialized kernels without importing their refinement proofs. It alone selects the exact word baseline when the partial pair kernel declines.

@[inline]

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

Use the fixed-pair square-root kernel when eligible, otherwise use the word dispatcher.

The partial pair result has one fallback call to word. The fixed-format guard lets specialization discard the pair probe for binary32 and binary64; neither is pair-eligible.