ocaml: phase 5.1 palindrome_sum.ml baseline (sum of 3-digit palindromes = 49500)

let palindrome_sum lo hi =
    let total = ref 0 in
    for n = lo to hi do
      if is_pal n then total := !total + n
    done;
    !total

  palindrome_sum 100 999 = 49500

There are 90 three-digit palindromes (form aba; 9 choices for a, 10
for b). Average value 550, sum 49500.

Companion to palindrome.ml (predicate-only) and paren_depth.ml.

127 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,10 @@ _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.
+- 2026-05-10 Phase 5.1 — palindrome_sum.ml baseline (sum of 3-digit
+  palindromes = 49500). 90 palindromes between 100 and 999 (form
+  aba; 9 choices for a, 10 for b). Sum = 49500 = 90 * 550 (mean
+  is 550). 127 baseline programs total.
 - 2026-05-10 Phase 5.1 — triangle_div.ml baseline (first triangle
   number with > 10 divisors = 120). PE12 with target 10. T(15) = 120
   has 16 divisors {1,2,3,4,5,6,8,10,12,15,20,24,30,40,60,120} —