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.

lib/ocaml/baseline/expected.json

@@ -4,6 +4,7 @@
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,
   "factorial.ml": 3628800,
+  "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "module_use.ml": 3,
   "option_match.ml": 5,