mk: graph reachability via patho — classic miniKanren

Defines a small graph as a fact list, edgeo for fact lookup, and patho
that recursively constructs paths. Direct-edge clause yields (x y);
otherwise traverse one edge to z, recurse for z->y, prepend x.

Enumerates all paths between two nodes, including alternates through
shortcut edges:
  (run* q (patho :a :d q))
    -> ((:a :b :c :d) (:a :c :d))    ; both routes

6 new tests, 316/316 cumulative.

plans/minikanren-on-sx.md

@@ -173,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **Graph reachability via patho**: classic miniKanren
+  graph search. `edgeo` looks up edges in a fact list via `membero`; `patho`
+  recursively builds paths via direct-edge OR (one edge + recurse + cons).
+  Enumerates all paths between two nodes, including alternates through
+  shortcuts. 6 new tests, 316/316 cumulative.
 - **2026-05-08** — **everyo / someo (predicate-style relations)**:
   `(everyo rel l)` — every element of l satisfies rel; `(someo rel l)` —
   some element does. Both compose with intarith for numeric tests: