mk: phase 6F — fd-distinct (pairwise alldifferent)

(fd-distinct (list a b c ...)) imposes pairwise distinctness via O(n²)
fd-neq constraints. Each fd-neq propagates independently when any pair
becomes ground or has a domain-removable value.

Tests: empty/singleton trivially succeed; pair-distinct/equal cover
correctness; 3-perms-of-3 = 6 and 4-perms-of-4 = 24 confirm full
permutation enumeration; pigeonhole 4-of-3 fails.

7 new tests, 610/610 cumulative.

lib/minikanren/clpfd.sx

@@ -459,3 +459,28 @@
     (cond
       ((empty? vars) succeed)
       (:else (mk-conj (fd-label-one (first vars)) (fd-label (rest vars)))))))
+
+;; --- fd-distinct (pairwise distinct via fd-neq) ---
+
+(define
+  fd-distinct-from-head
+  (fn
+    (x others)
+    (cond
+      ((empty? others) succeed)
+      (:else
+        (mk-conj
+          (fd-neq x (first others))
+          (fd-distinct-from-head x (rest others)))))))
+
+(define
+  fd-distinct
+  (fn
+    (vars)
+    (cond
+      ((empty? vars) succeed)
+      ((empty? (rest vars)) succeed)
+      (:else
+        (mk-conj
+          (fd-distinct-from-head (first vars) (rest vars))
+          (fd-distinct (rest vars)))))))