ocaml: phase 5.1 floyd_warshall.ml baseline (4-node APSP, dist(0,3)=9)

Floyd-Warshall all-pairs shortest path with triple-nested for-loop:

  for k = 0 to n - 1 do
    for i = 0 to n - 1 do
      for j = 0 to n - 1 do
        if d.(i).(k) + d.(k).(j) < d.(i).(j) then
          d.(i).(j) <- d.(i).(k) + d.(k).(j)
      done
    done
  done

Graph (4 nodes, directed):
  0->1 weight 5, 0->3 weight 10, 1->2 weight 3, 2->3 weight 1

Direct edge 0->3 = 10, but path 0->1->2->3 = 5+3+1 = 9.

Tests 2D array via Array.init with closure, nested .(i).(j) read
+ write, triple-nested for, in-place mutation under aliasing.

148 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -70,6 +70,7 @@
   "fizz_classifier.ml": 540,
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,
+  "floyd_warshall.ml": 9,
   "lis.ml": 6,
   "list_ops.ml": 30,
   "luhn.ml": 2,