ocaml: phase 5.1 gray_code.ml baseline (4-bit reflected Gray code, sum+len = 136)

Single-formula generation:

  gray[i] = i lxor (i lsr 1)

For n = 4, generates 16 values, each differing from its neighbour
by one bit. Output is a permutation of 0..15, so its sum equals the
natural-sequence sum 120; +16 entries -> 136.

Tests lsl / lxor / lsr together (the iter-127 bitwise ops) plus
Array.make / Array.fold_left.

95 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -30,6 +30,7 @@
   "fraction.ml": 7,
   "frequency.ml": 5,
   "gcd_lcm.ml": 60,
+  "gray_code.ml": 136,
   "grep_count.ml": 3,
   "grid_paths.ml": 210,
   "group_consec.ml": 53,