ocaml: phase 5.1 min_cost_path.ml baseline (4x4 grid DP, optimal cost 12)

Standard 2D DP for min-cost path with right/down moves only:

  dp[i][j] = min(dp[i-1][j], dp[i][j-1]) + cost[i][j]

  cost:                  dp:
    1 3 1 2                1  4  5  7
    1 5 1 3                2  7  6  9
    4 2 1 4                6  8  7 11
    1 6 2 3                7 13  9 12

Optimal cost from (0,0) to (3,3) = 12.

Tests nested 2D arrays via Array.init + Array.make, double-nested
for-loops with branched edges (first row, first column, general),
mixed .(i-1).(j) read + .(i).(j)<- write on the same DP array.

161 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -101,6 +101,7 @@
   "merge_intervals.ml": 12,
   "merge_sort.ml": 44,
   "merge_two.ml": 441,
+  "min_cost_path.ml": 12,
   "module_use.ml": 3,
   "monotonic.ml": 4,
   "newton_sqrt.ml": 1414,