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.

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — validate.ml baseline (Either-based input
+  validation, 3 errors × 100 + 117 sum = 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, 117) → 417. Exercises Either constructors used
+  bare (no qualification), char range comparison, tuple-pattern
+  destructuring on let-binding, recursive helper inside if-else. 36
+  baseline programs total.
 - 2026-05-09 Phase 5.1 — word_freq.ml baseline (Map.Make on String,
   count distinct words → 8). Defines a StringOrd module + applies
   Map.Make to it. Folds the input through SMap.find_opt + SMap.add to