ocaml: phase 5.1 csv.ml baseline (split + int_of_string + fold_left)

Inline CSV-like text:
  a,1,extra
  b,2,extra
  c,3,extra
  d,4,extra

Two-stage String.split_on_char: first on '\n' for rows, then on ','
for fields per row. List.fold_left accumulates int_of_string of the
second field across rows. Result = 1+2+3+4 = 10.

Exercises char escapes inside string literals ('\n'), nested
String.split_on_char, List.fold_left with a non-trivial closure body,
and int_of_string. 23 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -5,6 +5,7 @@
   "caesar.ml": 215,
   "calc.ml": 13,
   "closures.ml": 315,
+  "csv.ml": 10,
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,
   "factorial.ml": 3628800,