mk: sortedo — list is non-decreasing (intarith)

Three conde clauses: empty list / singleton list / two-or-more (where
the first two satisfy lteo-i and the rest is recursively sorted). Uses
ground-only integer comparison (intarith), so the input list must
walk to ground integers.

7 new tests, 463/463 cumulative.

lib/minikanren/tests/sortedo.sx

@@ -0,0 +1,40 @@
+;; lib/minikanren/tests/sortedo.sx — checks list is non-decreasing.
+
+(mk-test
+  "sortedo-empty"
+  (run* q (sortedo (list)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-singleton"
+  (run* q (sortedo (list 42)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-ascending"
+  (run* q (sortedo (list 1 2 3 4)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-with-equal-adjacent"
+  (run*
+    q
+    (sortedo (list 1 1 2 2 3)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "sortedo-out-of-order-fails"
+  (run* q (sortedo (list 1 3 2)))
+  (list))
+
+(mk-test
+  "sortedo-descending-fails"
+  (run* q (sortedo (list 3 2 1)))
+  (list))
+
+(mk-test
+  "sortedo-pair-equal"
+  (run* q (sortedo (list 5 5)))
+  (list (make-symbol "_.0")))
+
+(mk-tests-run!)