mk: phase 6A — minimal FD (ino + all-distincto)

ino is membero with the constraint-store-friendly argument order
(`(ino x dom)` reads as "x in dom"). all-distincto checks pairwise
distinctness via nafc + membero on the recursive tail. These two are
enough to express the enumerate-then-filter style of finite-domain
solving:

  (fresh (a b c)
    (ino a (list 1 2 3)) (ino b (list 1 2 3)) (ino c (list 1 2 3))
    (all-distincto (list a b c)))

enumerates all 6 distinct triples from {1, 2, 3}. Full CLP(FD) with
arc-consistency, fd-plus, etc. remains pending under Phase 6 proper.

9 new tests, 237/237 cumulative.

lib/minikanren/fd.sx

@@ -0,0 +1,25 @@
+;; lib/minikanren/fd.sx — Phase 6 piece A: minimal finite-domain helpers.
+;;
+;; A full CLP(FD) engine (arc consistency, native integer domains, fd-plus
+;; etc.) is Phase 6 proper. For now we expose two small relations layered
+;; on the existing list machinery — they're sufficient for permutation
+;; puzzles, the N-queens-style core of constraint solving:
+;;
+;;   (ino x dom)         — x is a member of dom (alias for membero with the
+;;                         constraint-store-friendly argument order).
+;;   (all-distincto l)   — all elements of l are pairwise distinct.
+;;
+;; all-distincto uses nafc + membero on the tail — it requires the head
+;; element of each recursive step to be ground enough for membero to be
+;; finitary, so order matters: prefer (in x dom) goals BEFORE
+;; (all-distincto (list x ...)) so values get committed first.
+
+(define ino (fn (x dom) (membero x dom)))
+
+(define
+  all-distincto
+  (fn
+    (l)
+    (conde
+      ((nullo l))
+      ((fresh (a d) (conso a d l) (nafc (membero a d)) (all-distincto d))))))