ocaml: phase 6 List.combine/split/iter2/fold_left2/map2 (+4, 367 total)

Mechanical pair-walk OCaml implementations. failwith on length
mismatch matches Stdlib semantics. List module now covers 30+
functions.

plans/ocaml-on-sx.md

@@ -250,7 +250,8 @@ SX CEK evaluator (both JS and OCaml hosts)
       `append`, `iter`, `for_all`, `exists`, `mem`, `nth`, `hd`, `tl`,
       `rev_append`, `concat`/`flatten`, `init`, `iteri`, `mapi`, `find`,
       `find_opt`, `assoc`, `assoc_opt`, `partition`, `sort`,
-      `stable_sort` (insertion sort, O(n²)). _(Pending: combine, split.)_
+      `stable_sort`, `combine`, `split`, `iter2`, `fold_left2`, `map2`.
+      30+ functions covered.
 - [~] `Option`: `map`, `bind`, `value`, `get`, `is_none`, `is_some`,
       `iter`, `fold`, `to_list`. _(Pending: join/to_result.)_
 - [~] `Result`: `map`, `bind`, `is_ok`, `is_error`, `get_ok`,
@@ -372,6 +373,10 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — List.combine/split/iter2/fold_left2/map2 (+4
+  tests, 367 total). Mechanical pair-walk OCaml implementations,
+  failwith on length-mismatch matching Stdlib semantics. List module
+  now covers 30+ functions.
 - 2026-05-08 Phase 5.1 — baseline expanded to 8 programs (8/8 pass).
   Added: closures.ml (curried adders), quicksort.ml (recursive sort
   on lists), exception_handle.ml (exception decl + raise + try/with).