First half of the #281 clock-enable set: Sign promotes the private
sign0() semantics to vocabulary (strict comparisons; NaN and -0.0
fall to 0.0), Select is the stateless 2:1 mux that consumes Bool
back into the F64 plane. Both zero-arg, Firing::Any, rostered
(count pins 29→31); an engine E2E drives both through the real
GraphBuilder→compile→run seam.
refs #281
