ocaml: phase 2+6 print primitives wire to host display (+2 tests, 444 total)

print_string / print_endline / print_int / print_newline now route to
SX display primitive (not the non-existent print/println). print_endline
appends '\n'.

let _ = expr ;; at top level confirmed working via the
wildcard-param parser.

plans/ocaml-on-sx.md

@@ -399,6 +399,13 @@ _Newest first._
   recognise `!` as the prefix-deref of an application argument, so
   `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
   uses a ref holding a string list; contents reverses and concats.
+- 2026-05-08 Phase 2+6 — print primitives wired to host `display` (+2
+  tests, 444 total). `print_string` / `print_endline` / `print_int` /
+  `print_newline` now use SX `display` (no auto-newline) plus an
+  explicit `"\n"` for endline. Prior version referenced `print`/
+  `println` host primitives that don't exist. `let _ = expr ;;`
+  top-level decl works as expected (already supported by the
+  wildcard-param parser).
 - 2026-05-08 Phase 5 — HM let-mut / let-rec-mut inference (+3 tests,
   442 total). `ocaml-infer-let-mut` infers each rhs in the parent env
   and generalizes sequentially; `ocaml-infer-let-rec-mut` pre-binds