mk: samelengtho — equal-length relation

Recurses positionally, dropping a head from each list each step. Both
arguments can be unbound, giving the natural enumeration:

  (run 3 q (fresh (l1 l2) (samelengtho l1 l2) (== q (list l1 l2))))
    -> (((), ())                         empty/empty
        ((_.0), (_.1))                   pair of 1-element lists
        ((_.0 _.1), (_.2 _.3)))          pair of 2-element lists

5 new tests, 293/293 cumulative.

plans/minikanren-on-sx.md

@@ -173,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **samelengtho**: classic miniKanren relation that
+  succeeds when two lists have equal length. Symmetric — works to enumerate
+  both lists fresh: `(run 3 q (fresh (l1 l2) (samelengtho l1 l2) (== q
+  (list l1 l2))))` produces empty/empty, then 1-elem pairs, then 2-elem.
+  5 new tests, 293/293 cumulative.
 - **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