ocaml: phase 5.1 trapping_rain.ml baseline (LeetCode trapped water = 6)

Classic trapped-rain-water two-pass DP:

  left_max[i]  = max(heights[0..i])    (forward sweep)
  right_max[i] = max(heights[i..n-1])  (downto sweep)
  water        = sum over i of (min(left_max[i], right_max[i])
                                  - heights[i])

For [0; 1; 0; 2; 1; 0; 1; 3; 2; 1; 2; 1]: water = 6.

Tests dual sweep (forward + downto), array of running maxes,
inline-if rhs of <- for running-max update (uses iter-236 fix
for <- accepting if/match RHS).

203 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -194,6 +194,7 @@
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "tree_depth.ml": 4,
+  "trapping_rain.ml": 6,
   "triangle.ml": 11,
   "trie.ml": 6,
   "triangle_div.ml": 120,