mk: pythagorean triples search — intarith showcase

Finds all (a, b, c) with a, b, c in [1..10], a <= b, a^2 + b^2 = c^2.
Result: ((3 4 5) (6 8 10)) — the two smallest Pythagorean triples
within the domain.

Demonstrates the enumerate-then-filter pattern:
  (ino a dom) (ino b dom) (ino c dom) — generate
  (lteo-i a b) — symmetry break
  (*o-i a a a-sq) (*o-i b b b-sq) (*o-i c c c-sq) — squares
  (pluso-i a-sq b-sq sum) (== sum c-sq) — Pythagorean equation

288/288 cumulative.

plans/minikanren-on-sx.md

@@ -173,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **Pythagorean triples (intarith showcase)**: search for
+  (a, b, c) ∈ [1..10]³, a ≤ b, a² + b² = c² via `ino + lteo-i + *o-i +
+  pluso-i + ==`. Finds exactly `(3 4 5)` and `(6 8 10)`. Demonstrates the
+  enumerate-then-filter pattern with intarith escape into host arithmetic.
+  1 new test, 288/288 cumulative.
 - **2026-05-08** — **intarith.sx — fast ground-only integer arithmetic**:
   pluso-i / minuso-i / *o-i / lto-i / lteo-i / neqo-i wrap host arithmetic
   via `project`. They are not relational like Peano `pluso` (require args