ocaml: phase 5.1 tic_tac_toe.ml baseline (3x3 winner check, X wins top row = 1)

Board as 9-element flat int array, 0=empty, 1=X, 2=O. Three
predicate functions:

  check_row b r       check_col b c       check_diag b

each return the winning player's mark or 0. Main 'winner' loops
i = 0..2 calling row(i)/col(i) then check_diag, threading via a
result ref.

Test board:
  X X X
  . O .
  . . O

X wins on row 0 -> winner returns 1.

Tests Array.of_list with row-major 'b.(r * 3 + c)' indexing,
multi-fn collaboration, and structural equality on int values.

66 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 — tic_tac_toe.ml baseline (3x3 winner check,
+  X wins top row → 1). Board encoded as 9-element flat int array
+  with 0=empty, 1=X, 2=O. Three predicate functions check row,
+  column, and either diagonal; main `winner` loops over the 8
+  winning lines. Tests Array.of_list with row-major indexing,
+  multi-fn collaboration, and structural equality on int values.
+  66 baseline programs total.
 - 2026-05-09 Phase 5.1 — subset_sum.ml baseline (count subsets of
   [1..8] summing to 10 = 8). Pure recursion: at each element, take
   it or don't. Base case: target=0 → 1, target≠0 → 0. 2^8 = 256