ocaml: phase 5.1 memo_fib.ml baseline (16/16 pass)

Memoized fibonacci using Hashtbl.find_opt + Hashtbl.add.
fib(25) = 75025. Demonstrates mutable Hashtbl through the OCaml
stdlib API in real recursive code.

plans/ocaml-on-sx.md

@@ -407,6 +407,9 @@ _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.
+- 2026-05-08 Phase 5.1 — memo_fib.ml baseline (16/16 pass). Memoized
+  fibonacci using `Hashtbl.find_opt` + `Hashtbl.add`. fib(25) = 75025.
+  Demonstrates mutable dict semantics through the OCaml stdlib API.
 - 2026-05-08 Phase 5.1 — queens.ml baseline (15/15 pass). 4-queens
   count via recursive backtracking with `List.fold_left`. Returns 2
   (the two solutions of 4-queens). Per-program timeout in run.sh