lib/minikanren/unify.sx wraps lib/guest/match.sx with a miniKanren-flavoured
cfg: native SX lists as cons-pairs, occurs-check off by default. ~22 lines
of local logic over kit's walk-with / unify-with / extend / occurs-with.
48 tests in lib/minikanren/tests/unify.sx exercise: var fresh-distinct,
walk chains, walk* deep into nested lists, atom/var/list unification with
positional matching, failure modes, opt-in occurs check.