ocaml: phase 6 stdlib slice (List/Option/Result, +23 tests, 248 total)

lib/ocaml/runtime.sx defines the stdlib in OCaml syntax (not SX): every
function exercises the parser, evaluator, match engine, and module
machinery built in earlier phases. Loaded once via ocaml-load-stdlib!,
cached in ocaml-stdlib-env, layered under user code via ocaml-base-env.

List: length, rev, rev_append, map, filter, fold_left/right, append,
iter, mem, for_all, exists, hd, tl, nth.
Option: map, bind, value, get, is_none, is_some.
Result: map, bind, is_ok, is_error.

Substrate validation: this stdlib is a nontrivial OCaml program — its
mere existence proves the substrate works.

plans/ocaml-on-sx.md

@@ -215,14 +215,14 @@ SX CEK evaluator (both JS and OCaml hosts)
 
 ### Phase 6 — Standard library
 
-- [ ] `List`: `map`, `filter`, `fold_left`, `fold_right`, `length`, `rev`, `append`,
-      `concat`, `flatten`, `iter`, `iteri`, `mapi`, `for_all`, `exists`, `find`,
-      `find_opt`, `mem`, `assoc`, `assq`, `sort`, `stable_sort`, `nth`, `hd`, `tl`,
-      `init`, `combine`, `split`, `partition`.
-- [ ] `Option`: `map`, `bind`, `fold`, `get`, `value`, `join`, `iter`, `to_list`,
-      `to_result`, `is_none`, `is_some`.
-- [ ] `Result`: `map`, `bind`, `fold`, `get_ok`, `get_error`, `map_error`,
-      `to_option`, `is_ok`, `is_error`.
+- [~] `List`: `map`, `filter`, `fold_left`, `fold_right`, `length`, `rev`,
+      `append`, `iter`, `for_all`, `exists`, `mem`, `nth`, `hd`, `tl`,
+      `rev_append`. _(Pending: concat/flatten, iteri/mapi, find/find_opt,
+      assoc/assq, sort, init, combine, split, partition.)_
+- [~] `Option`: `map`, `bind`, `value`, `get`, `is_none`, `is_some`.
+      _(Pending: fold/join/iter/to_list/to_result.)_
+- [~] `Result`: `map`, `bind`, `is_ok`, `is_error`. _(Pending:
+      fold/get_ok/get_error/map_error/to_option.)_
 - [ ] `String`: `length`, `get`, `sub`, `concat`, `split_on_char`, `trim`,
       `uppercase_ascii`, `lowercase_ascii`, `contains`, `starts_with`, `ends_with`,
       `index_opt`, `replace_all` (non-stdlib but needed).
@@ -327,6 +327,16 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — `lib/ocaml/runtime.sx` minimal stdlib slice
+  written entirely in OCaml syntax: List (length, rev, rev_append, map,
+  filter, fold_left/right, append, iter, mem, for_all, exists, hd, tl,
+  nth), Option (map, bind, value, get, is_none, is_some), Result (map,
+  bind, is_ok, is_error). Loaded once via `ocaml-load-stdlib!`, cached
+  in `ocaml-stdlib-env`; `ocaml-run` and `ocaml-run-program` layer user
+  code on top via `ocaml-base-env`. The fact that these are written in
+  OCaml (not SX) and parse + evaluate cleanly is a substrate-validation
+  win: every parser, eval, match, ref, and module path proven by a
+  single nontrivial Ocaml program. 248/248 (+23).
 - 2026-05-08 Phase 4 — functors + module aliases (+5 tests, 225 total).
   Parser: `module F (M) = struct DECLS end` → `(:functor-def NAME PARAMS
   DECLS)`. `module N = expr` (where expr isn't `struct`) → `(:module-alias