ocaml: phase 5.1 radix_sort.ml baseline (LSD radix sort, sentinel 802002)

LSD radix sort over base 10 digits. Per pass:
  - 10 bucket-refs created via Array.init 10 (fun _ -> ref []) (each
    closure call yields a distinct list cell)
  - scan array, append each value to its digit's bucket
  - flatten buckets back to the array in order

Input  [170;45;75;90;802;24;2;66]
Output [2;24;45;66;75;90;170;802]

Sentinel: a.(0) + a.(7)*1000 = 2 + 802*1000 = 802002.

Tests array-of-refs with !buckets.(d) deref, list-mode bucket
sort within in-place array sort, unused for-loop var (`for _ =
1 to maxd`).

159 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -127,6 +127,7 @@
   "queens.ml": 2,
   "quickselect.ml": 5,
   "quicksort.ml": 44,
+  "radix_sort.ml": 802002,
   "roman.ml": 44,
   "rolling_hash.ml": 6,
   "reverse_int.ml": 54329,