mk: flatteno — nested list flattener

Three conde clauses: nullo tree -> nullo flat; pairo tree -> recurse on
car & cdr, appendo their flattenings; otherwise tree must be a ground
non-list atom (nafc nullo + nafc pairo) and flat = (list tree).

Works on ground inputs of arbitrary nesting:
  (run* q (flatteno (list 1 (list 2 3) (list (list 4) 5)) q))
    -> ((1 2 3 4 5))

7 tests, 253/253 cumulative. Phase 4 list relations now complete.

plans/minikanren-on-sx.md

@@ -111,7 +111,11 @@ Key semantic mappings:
 - [x] `caro` / `cdro` / `conso` / `firsto` / `resto`
 - [x] `reverseo` `l` `r` — reverse of list. Forward is fast; backward is `run 1`-clean,
       `run*` diverges due to interleaved unbounded list search (canonical TRS issue).
-- [ ] `flatteno` `l` `f` — flatten nested lists (deferred — needs atom predicate)
+- [x] `flatteno` `l` `f` — flatten nested lists. Three conde clauses:
+      empty → empty; pair → recurse on car & cdr + appendo; otherwise atom →
+      `(== flat (list tree))`. Atom-ness is asserted via `nafc (nullo tree)
+      + nafc (pairo tree)` — both succeed only when tree is a ground non-list.
+      Works on ground inputs.
 - [x] `permuteo` `l` `p` — permutation of list. Built on `inserto` (insert at any
       position) and recursive permutation of tail. All 6 perms of (1 2 3) generated
       forward; backward `run 1 q (permuteo q (a b c))` finds the input.
@@ -169,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **flatteno**: nested-list flattener via conde + appendo.
+  Atom-ness is detected via `nafc (nullo ...) + nafc (pairo ...)` so the
+  third clause only fires when the input is a ground non-list. Works for
+  `()` / atoms / flat lists / arbitrarily-nested lists. 7 new tests,
+  253/253 cumulative. Phase 4 list relations all complete.
 - **2026-05-08** — **Laziness tests**: explicitly verifies the
   Zzz-on-conde-clauses + mk-mplus-swap-on-paused machinery: an
   infinitely-recursive relation truncated via `run 4 q (listo-aux q)`