mk: 4-queens classic benchmark green

queens.sx encodes a queen in row i at column ci. ino-each constrains
each ci to {1..n}; all-distincto handles the row/column distinct
property; safe-diag uses project to escape into host arithmetic for the
|c_i - c_j| != |i - j| diagonal guard. all-cells-safe iterates pairs at
goal-construction time so the constraint set is materialised once,
then driven by the search.

  (run* q (fresh (a b c d) (== q (list a b c d))
                            (queens-cols (list a b c d) 4)))
    -> ((2 4 1 3) (3 1 4 2))

Both valid 4-queens placements found. 6 new tests including the
two-solution invariant; 243/243 cumulative.

plans/minikanren-on-sx.md

@@ -169,6 +169,14 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **N-queens (classic miniKanren benchmark green)**:
+  `lib/minikanren/queens.sx`. Encoding cols(i) = column of queen in row i;
+  `ino-each` + `all-distincto` cover row/column constraints; `safe-diag`
+  uses `project` to escape into host arithmetic for the `|c_i - c_j| ≠
+  |i - j|` diagonal check; `all-cells-safe` walks pairs at construction
+  time. `(run* q (fresh (a b c d) (== q (list a b c d)) (queens-cols
+  (list a b c d) 4)))` returns the two valid 4-queens placements
+  `(2 4 1 3)` and `(3 1 4 2)`. 6 new tests, 243/243 cumulative.
 - **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