ocaml: phase 5.1 euler4_small.ml baseline (largest 2-digit palindrome product = 9009)

Scaled-down Project Euler #4. Real version uses 3-digit numbers
yielding 906609 = 913 * 993; that's an 810k-iteration nested loop
that times out under our contended-host spec-level evaluator.

The 2-digit version (10..99) is fast enough and tests the same
algorithm:
  9009 = 91 * 99   (the only 2-digit-product palindrome > 9000)

Implementation:
  is_pal n     index-walk comparing s.[i] to s.[len-1-i]
  euler4 lo hi nested for with running max + early-skip via
                'p > !m && is_pal p' short-circuit

111 baseline programs total.

lib/ocaml/baseline/euler4_small.ml

@@ -0,0 +1,21 @@
+let is_pal n =
+  let s = string_of_int n in
+  let len = String.length s in
+  let p = ref true in
+  for i = 0 to len / 2 - 1 do
+    if s.[i] <> s.[len - 1 - i] then p := false
+  done;
+  !p
+
+let euler4 lo hi =
+  let m = ref 0 in
+  for a = lo to hi do
+    for b = a to hi do
+      let p = a * b in
+      if p > !m && is_pal p then m := p
+    done
+  done;
+  !m
+;;
+
+euler4 10 99

lib/ocaml/baseline/expected.json

@@ -28,6 +28,7 @@
   "euler1.ml": 233168,
   "euler10.ml": 1060,
   "euler2.ml": 4613732,
+  "euler4_small.ml": 9009,
   "euler5.ml": 232792560,
   "euler6.ml": 25164150,
   "euler9.ml": 31875000,

plans/ocaml-on-sx.md

@@ -407,6 +407,11 @@ _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 — euler4_small.ml baseline (largest 2-digit
+  palindrome product = 9009 = 91 * 99). Scaled-down Project Euler
+  #4 (real version uses 3-digit numbers, 906609; that's 810k inner
+  iterations and would time out under contention). Tests palindrome
+  predicate via index-walk + nested for. 111 baseline programs total.
 - 2026-05-09 Phase 5.1 — euler10.ml baseline (sum of primes ≤ 100 =
   1060, scaled-down Project Euler #10). Sieve of Eratosthenes
   followed by a sum loop. Used 100 instead of 2 million to fit our