ocaml: phase 5.1 tree_depth.ml baseline (binary tree depth, longest path = 4)

Same 'tree = Leaf | Node of int * tree * tree' ADT as iter-159
max_path_tree.ml, but the recursion ignores the value:

  let rec depth t = match t with
    | Leaf -> 0
    | Node (_, l, r) ->
      let dl = depth l in
      let dr = depth r in
      1 + (if dl > dr then dl else dr)

For the test tree:
        1
       /       2   3
     /     4   5
       /
      8

longest path is 1 -> 2 -> 5 -> 8, depth = 4.

Tests wildcard pattern in constructor 'Node (_, l, r)', two nested
let-bindings in match arm, inline if-as-expression for max.

83 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.
+- 2026-05-09 Phase 5.1 — tree_depth.ml baseline (binary tree depth,
+  longest path = 4). Same `tree = Leaf | Node of int * tree * tree`
+  ADT as iter 159, but recursion now ignores the value
+  (`Node (_, l, r)`) and returns `1 + max (depth l) (depth r)`. Uses
+  wildcard pattern in constructor, two nested let-bindings in arm,
+  and inline if-as-expression for max. 83 baseline programs total.
 - 2026-05-09 Phase 5.1 — stable_unique.ml baseline (Hashtbl-tracked
   dedupe preserving order, length+sum = 8+38 = 46). Walks input
   with `Hashtbl.mem` + `Hashtbl.add seen x ()` (unit-payload to use