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,

lib/ocaml/baseline/fizzbuzz.ml

@@ -0,0 +1,17 @@
+(* Baseline: fizzbuzz returning a list of strings *)
+let fizzbuzz n =
+  let acc = ref [] in
+  for i = 1 to n do
+    let s =
+      if i mod 15 = 0 then "FizzBuzz"
+      else if i mod 3 = 0 then "Fizz"
+      else if i mod 5 = 0 then "Buzz"
+      else Int.to_string i
+    in
+    acc := s :: !acc
+  done ;
+  List.rev !acc
+;;
+
+(* Concatenated for a deterministic check value via String.length *)
+String.length (String.concat "," (fizzbuzz 15))