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.

lib/ocaml/baseline/expected.json

@@ -7,6 +7,7 @@
   "factorial.ml": 3628800,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
+  "memo_fib.ml": 75025,
   "module_use.ml": 3,
   "mutable_record.ml": 10,
   "option_match.ml": 5,

lib/ocaml/baseline/memo_fib.ml

@@ -0,0 +1,15 @@
+(* Baseline: memoized fibonacci using Hashtbl *)
+let cache = Hashtbl.create 16 ;;
+
+let rec fib n =
+  if n < 2 then n
+  else
+    match Hashtbl.find_opt cache n with
+    | Some v -> v
+    | None ->
+      let v = fib (n - 1) + fib (n - 2) in
+      Hashtbl.add cache n v ;
+      v
+;;
+
+fib 25