mk: not-membero — relational "x is not in l"

Mirrors the structure all-distincto already uses internally: walk the
list, ensure each element is not equal to x via nafc, recurse on tail.
Useful as a constraint-style filter:

  (membero x (list 1 2 3 4 5))
  (not-membero x (list 2 4))
    -> x in {1, 3, 5}

4 new tests, 382/382 cumulative.

plans/minikanren-on-sx.md

@@ -173,6 +173,10 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **not-membero**: relational "x is not a member of l".
+  Uses `nafc + ==` per element (the same skeleton all-distincto uses).
+  Useful as a constraint-style filter: `(membero x dom) (not-membero x
+  excluded)`. 4 new tests, 382/382 cumulative.
 - **2026-05-08** — **repeato + concato**: repeato builds a list of n copies
   (Peano n); concato is fold-appendo over a list of lists. Both run forward
   and backward — repeato can recover the count from a uniform list. 10 new