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/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,