ocaml: phase 6 String/Char/Int/Float/Printf modules (+13 tests, 278 total)

Host primitives _string_length / _string_sub / _char_code / etc. exposed
in the base env (underscore-prefixed to avoid user clash). lib/ocaml/
runtime.sx wraps them into OCaml-syntax modules: String (length, get,
sub, concat, uppercase/lowercase_ascii, starts_with), Char (code, chr,
lowercase/uppercase_ascii), Int (to_string, of_string, abs, max, min),
Float.to_string, Printf stubs.

Also added print_string / print_endline / print_int IO builtins.

plans/ocaml-on-sx.md

@@ -229,6 +229,16 @@ SX CEK evaluator (both JS and OCaml hosts)
       _(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`, `uppercase_ascii`,
+      `lowercase_ascii`, `starts_with`. _(Pending: split_on_char, trim,
+      contains, ends_with, index_opt, replace_all.)_
+- [~] `Char`: `code`, `chr`, `lowercase_ascii`, `uppercase_ascii`.
+      _(Pending: escaped.)_
+- [~] `Int`: `to_string`, `of_string`, `abs`, `max`, `min`.
+      _(Pending: arithmetic helpers, min_int/max_int.)_
+- [~] `Float`: `to_string`. _(Pending: of_string, arithmetic helpers.)_
+- [~] `Printf`: stub `sprintf`/`printf`. _(Real format-string
+      interpretation pending.)_
 - [ ] `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).
@@ -333,6 +343,14 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — extended stdlib slice (+13 tests, 278 total).
+  Host primitives exposed via `_string_*`, `_char_*`, `_int_*`,
+  `_string_of_*` underscore-prefixed builtins so the OCaml-side
+  `lib/ocaml/runtime.sx` modules can wrap them: String (length, get,
+  sub, concat, uppercase_ascii, lowercase_ascii, starts_with), Char
+  (code, chr, lowercase_ascii, uppercase_ascii), Int (to_string,
+  of_string, abs, max, min), Float.to_string, Printf stubs. Also added
+  `print_string`/`print_endline`/`print_int` builtins.
 - 2026-05-08 Phase 5 — Hindley-Milner type inference, paired-sequencing
   consumer of `lib/guest/hm.sx` (algebra) and `lib/guest/match.sx`
   (unify). `lib/ocaml/infer.sx` ships Algorithm W rules for OCaml AST: