ocaml: phase 5.1 triangle.ml baseline (Pascal-shape min path sum, 2+3+5+1 = 11)

Bottom-up DP minimum-path through a triangle:

       2
      3 4
     6 5 7
    4 1 8 3

  let min_path_triangle rows =
    initialise dp from last row;
    for r = n - 2 downto 0 do
      for c = 0 to row_len - 1 do
        dp.(c) <- row.(c) + min(dp.(c), dp.(c+1))
      done
    done;
    dp.(0)

The optimal path 2 -> 3 -> 5 -> 1 sums to 11.

Tests downto loop, Array.of_list inside loop body, nested arr.(i)
reads + writes, and inline if-then-else for min.

74 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -70,6 +70,7 @@
   "zip_unzip.ml": 1000,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
+  "triangle.ml": 11,
   "twosum.ml": 5,
   "unique_set.ml": 9,
   "validate.ml": 417,