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.

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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 — gray_code.ml baseline (4-bit binary
+  reflected Gray code, sum 120 + length 16 = 136). Single-formula
+  generation: `gray[i] = i lxor (i lsr 1)`. Outputs a permutation of
+  0..15, so its sum is the same 120 as the natural sequence; the
+  length-16 confirms 2^4 entries. Tests `lsl`/`lxor`/`lsr` together
+  and Array.make + Array.fold_left. 95 baseline programs total.
 - 2026-05-09 Phase 5.1 — max_run.ml baseline (longest consecutive
   run, sum of three test cases = 4+1+0 = 5). Walks list with
   `Some y when y = x` guard pattern in match for the prev-value