ocaml: phase 5.1 segment_tree.ml baseline (range-sum tree, fingerprint 4232)

Power-of-two-indexed segment tree over [1;3;5;7;9;11;13;15]:
  build sums (root holds total = 64)
  query returns range sum in O(log n)
  update propagates a point delta back up the path

Sequence:
  r1 = query [2,5] = 5 + 7 + 9 + 11 = 32
  update idx 3 += 10  (so a[3] becomes 17)
  r2 = query [2,5] = 5 + 17 + 9 + 11 = 42
  encoded fingerprint = r1 + r2*100 = 32 + 4200 = 4232

Tests three mutually independent recursive functions with array
index arithmetic on 2*node / 2*node+1, half-bisection on mid =
(l+r)/2, bottom-up combine pattern.

167 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -141,6 +141,7 @@
   "run_decode.ml": 21,
   "run_length.ml": 11,
   "safe_div.ml": 20,
+  "segment_tree.ml": 4232,
   "shuffle.ml": 55,
   "simpson_int.ml": 10000,
   "stable_unique.ml": 46,