ocaml: phase 5.1 magic_square.ml baseline (5x5 Siamese, diag sum = 65)

Siamese construction for odd-order magic squares:

  - place 1 at (0, n/2)
  - for k = 2..n^2, move up-right with (x-1+n) mod n wrap
  - if the target cell is taken, drop down one row instead

  for n=5, magic constant = n*(n^2+1)/2 = 5*26/2 = 65

Returns the main-diagonal sum (65 by construction).

Tests 2D array via Array.init + Array.make, mod arithmetic with
the (x-1+n) mod n idiom for negative-safe wrap, nested begin/end
branches inside for-loop body.

166 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -81,6 +81,7 @@
   "lis.ml": 6,
   "list_ops.ml": 30,
   "luhn.ml": 2,
+  "magic_square.ml": 65,
   "mat_mul.ml": 621,
   "matrix_power.ml": 832040,
   "max_path_tree.ml": 11,