ocaml: phase 6 String extensions (+6 tests, 406 total)

ends_with, contains, trim, split_on_char, replace_all, index_of —
wrap host SX primitives via new _string_* builtins. String module
now substantively covers OCaml's Stdlib.String.

plans/ocaml-on-sx.md

@@ -262,9 +262,9 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [~] `Hashtbl`: `create`, `add`, `find`, `find_opt`, `replace`, `mem`,
       `length`. Backed by a one-element list cell holding a SX dict;
       keys coerced to strings via `str` for polymorphic-key support.
-- [~] `String`: `length`, `get`, `sub`, `concat`, `uppercase_ascii`,
-      `lowercase_ascii`, `starts_with`. _(Pending: split_on_char, trim,
-      contains, ends_with, index_opt, replace_all.)_
+- [x] `String`: `length`, `get`, `sub`, `concat`, `uppercase_ascii`,
+      `lowercase_ascii`, `starts_with`, `ends_with`, `contains`, `trim`,
+      `split_on_char`, `replace_all`, `index_of`.
 - [~] `Char`: `code`, `chr`, `lowercase_ascii`, `uppercase_ascii`.
       _(Pending: escaped.)_
 - [~] `Int`: `to_string`, `of_string`, `abs`, `max`, `min`.
@@ -377,6 +377,9 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — String extensions: ends_with/contains/trim/
+  split_on_char/replace_all/index_of (+6 tests, 406 total). Wraps host
+  primitives via `_string_*` builtins.
 - 2026-05-08 Phase 6 — `List.take/drop/filter_map/flat_map/concat_map`
   (+6 tests, 400 total). Common functional helpers, all written in
   OCaml. **400-test milestone.**