Ground-cases propagator: when at least two of {x, y, z} walk to
ground numbers, the third is derived (or checked, if also ground).
Three vars with domains: deferred — no bounds-consistency in this
iteration.
Includes a small fd-bind-or-narrow helper that handles the common
"bind a var to a target int, respecting any existing domain"
pattern shared across propagators.
7 new tests: ground/ground/ground, recover x, recover y, impossible
case, domain-check rejection, x+y=5 enumeration, large numbers.
617/617 cumulative.
giles
5d3c248fdd