ocaml: phase 5.1 fizzbuzz.ml baseline (12/12 pass)

Classic fizzbuzz using ref-cell accumulator, for-loop, mod, if/elseif
chain, String.concat, Int.to_string. Output verified via String.length
of the comma-joined result for n=15: 57.

plans/ocaml-on-sx.md

@@ -399,6 +399,9 @@ _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 5.1 — fizzbuzz.ml baseline (12/12 pass). Classic
+  fizzbuzz using ref-cell accumulator, for-loop, mod, if/elseif chain,
+  String.concat, Int.to_string. Verifies output via String.length.
 - 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