ocaml: phase 5.1 btree.ml baseline (13/13 pass)

Polymorphic binary search tree with insert + in-order traversal.
Exercises parametric ADT (type 'a tree = Leaf | Node of 'a * 'a tree
* 'a tree), recursive match, List.append, List.fold_left.

plans/ocaml-on-sx.md

@@ -399,6 +399,10 @@ _Newest first._
   recognise `!` as the prefix-deref of an application argument, so
   `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
   uses a ref holding a string list; contents reverses and concats.
+- 2026-05-08 Phase 5.1 — btree.ml baseline (13/13 pass). Polymorphic
+  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.
 - 2026-05-08 Phase 5.1 — fizzbuzz.ml baseline (12/12 pass). Classic
   fizzbuzz using ref-cell accumulator, for-loop, mod, if/elseif chain,
   String.concat, Int.to_string. Verifies output via String.length.