mk: intarith.sx — ground-only integer arithmetic via project

pluso-i / minuso-i / *o-i / lto-i / lteo-i / neqo-i wrap host arithmetic
in project. They run at native speed but require their inputs to walk
to ground numbers — they are NOT relational the way Peano pluso is.
Use them when puzzle size makes Peano impractical (which is most cases
beyond toy examples).

Composes with relational goals — for instance,
  (fresh (x) (membero x (1 2 3 4 5)) (lto-i x 3) (== q x))
filters the domain by < 3 and returns (1 2).

18 new tests, 287/287 cumulative.

lib/minikanren/tests/intarith.sx

@@ -0,0 +1,89 @@
+;; lib/minikanren/tests/intarith.sx — ground-only integer arithmetic
+;; goals that escape into host operations via project.
+
+;; --- pluso-i ---
+
+(mk-test
+  "pluso-i-forward"
+  (run* q (pluso-i 7 8 q))
+  (list 15))
+(mk-test
+  "pluso-i-zero"
+  (run* q (pluso-i 0 0 q))
+  (list 0))
+(mk-test
+  "pluso-i-negatives"
+  (run* q (pluso-i -5 3 q))
+  (list -2))
+(mk-test
+  "pluso-i-non-ground-fails"
+  (run* q (fresh (a) (pluso-i a 3 5)))
+  (list))
+
+;; --- minuso-i ---
+
+(mk-test
+  "minuso-i-forward"
+  (run* q (minuso-i 10 4 q))
+  (list 6))
+(mk-test
+  "minuso-i-zero"
+  (run* q (minuso-i 5 5 q))
+  (list 0))
+
+;; --- *o-i ---
+
+(mk-test
+  "times-i-forward"
+  (run* q (*o-i 6 7 q))
+  (list 42))
+(mk-test
+  "times-i-by-zero"
+  (run* q (*o-i 0 99 q))
+  (list 0))
+(mk-test
+  "times-i-by-one"
+  (run* q (*o-i 1 17 q))
+  (list 17))
+
+;; --- comparisons ---
+
+(mk-test
+  "lto-i-true"
+  (run 1 q (lto-i 2 5))
+  (list (make-symbol "_.0")))
+(mk-test "lto-i-false" (run* q (lto-i 5 2)) (list))
+(mk-test "lto-i-equal-false" (run* q (lto-i 3 3)) (list))
+
+(mk-test
+  "lteo-i-equal"
+  (run 1 q (lteo-i 4 4))
+  (list (make-symbol "_.0")))
+(mk-test
+  "lteo-i-less"
+  (run 1 q (lteo-i 1 4))
+  (list (make-symbol "_.0")))
+(mk-test "lteo-i-more" (run* q (lteo-i 9 4)) (list))
+
+(mk-test
+  "neqo-i-different"
+  (run 1 q (neqo-i 3 5))
+  (list (make-symbol "_.0")))
+(mk-test "neqo-i-same" (run* q (neqo-i 3 3)) (list))
+
+;; --- composition with relational vars ---
+
+(mk-test
+  "intarith-with-membero"
+  (run*
+    q
+    (fresh
+      (x)
+      (membero
+        x
+        (list 1 2 3 4 5))
+      (lto-i x 3)
+      (== q x)))
+  (list 1 2))
+
+(mk-tests-run!)