ocaml: phase 5.1 validate.ml baseline (Either-based validation, 3 errs * 100 + 117 = 417)

validate_int returns Left msg on empty / non-digit, Right
(int_of_string s) on a digit-only string. process folds inputs with a
tuple accumulator (errs, sum), branching on the result.

  ['12'; 'abc'; '5'; ''; '100'; 'x']
  -> 3 errors (abc, '', x), valid sum = 12+5+100 = 117
  -> errs * 100 + sum = 417

Exercises:
  - Either constructors used bare (Left/Right without 'Either.'
    qualification)
  - char range comparison: c >= '0' && c <= '9'
  - tuple-pattern destructuring on let-binding (iter 98)
  - recursive helper defined inside if-else
  - List.fold_left with tuple accumulator

36 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -35,5 +35,6 @@
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "unique_set.ml": 9,
+  "validate.ml": 417,
   "word_count.ml": 3
 }