mk: 2x2 Latin square — small classic FD constraint demo

Defines latin-2x2 over 4 cells and 4 all-distincto constraints. Enumerates
exactly 2 squares ((1 2)(2 1)) and ((2 1)(1 2)); a corner clue narrows to
one. 3 new tests, 269/269 cumulative.

3x3 (12 squares, the natural showcase) is too slow under naive enumerate-
then-filter — that is the motivating test for Phase 6 arc-consistency.

plans/minikanren-on-sx.md

@@ -173,6 +173,12 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **2x2 Latin square**: small classic constraint demo using
+  `ino` + 4 `all-distincto` constraints. Enumerates exactly 2 squares
+  (`((1 2)(2 1))` and `((2 1)(1 2))`); a clue (top-left = 1) narrows to one.
+  3 new tests, 269/269 cumulative. Note: 3x3 (12 squares) is the natural
+  showcase but too slow under naive enumerate-then-filter — needs Phase 6
+  arc-consistency.
 - **2026-05-08** — **rembero / assoco / nth-o**: more standard list relations.
   rembero removes the first occurrence (uses `nafc (== a x)` to gate the
   skip clause, so it's well-defined on ground lists). assoco is alist