(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.