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.

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-11 Phase 5.1 — trapping_rain.ml baseline (trapped rain
+  water over heights [0;1;0;2;1;0;1;3;2;1;2;1] = 6). Left-max and
+  right-max prefix arrays; at each index water = min(L,R) − h.
+  Tests dual sweep (forward + downto), array of running maxes,
+  inline-if rhs of `<-` for running-max update (uses iter-236
+  fix). 203 baseline programs total.
 - 2026-05-11 Phase 5.1 — gas_station.ml baseline (find unique start
   station for circular gas tour, gas=[1;2;3;4;5] cost=[3;4;5;1;2]
   → start at index 3). Classic O(n) greedy: walk once tracking the