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.

plans/minikanren-on-sx.md

@@ -142,7 +142,10 @@ Key semantic mappings:
 
 ### Phase 6 — arithmetic constraints CLP(FD)
 - [ ] Finite domain variables: `fd-var` with domain `[lo..hi]`
-- [ ] `in` `x` `domain` — constrain x to domain
+- [x] `ino` `x` `domain` — alias for `(membero x domain)` with the
+      constraint-store-friendly argument order. Sufficient for the
+      enumerate-then-filter style of finite-domain solving.
+- [x] `all-distincto` `l` — pairwise-distinct elements via `nafc + membero`.
 - [ ] `fd-eq` `x` `y` — x = y (constraint propagation)
 - [ ] `fd-neq` `x` `y` — x ≠ y
 - [ ] `fd-lt` `fd-lte` `fd-gt` `fd-gte` — ordering constraints
@@ -166,6 +169,13 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **Phase 6 piece A — minimal FD (ino + all-distincto)**:
+  `lib/minikanren/fd.sx`. `ino` is `membero` with the FD-style argument
+  order; `all-distincto` is `nafc + membero` walking the list. Together
+  they cover the enumerate-then-filter style of finite-domain solving —
+  `(fresh (a b c) (ino a D) (ino b D) (ino c D) (all-distincto (list a b c)))`
+  enumerates all distinct triples from D. Full FD with arc-consistency,
+  fd-plus etc. is still pending. 9 new tests, 237/237 cumulative.
 - **2026-05-08** — **Classic puzzles + matche keyword fix**: matche now emits
   keywords bare in the pattern->expr conversion so they self-evaluate to their
   string name and unify with the same-keyword target value (instead of becoming