ocaml: phase 5.1 frequency.ml baseline + Format module alias (+2 tests, 498 total)

frequency.ml exercises the recently-added Hashtbl.iter / fold +
Hashtbl.find_opt + s.[i] indexing + for-loop together: build a
char-count table for 'abracadabra' then take the max via
Hashtbl.fold. Expected = 5 (a x 5). Total 25 baseline programs.

Format module added as a thin alias of Printf — sprintf, printf, and
asprintf all delegate to Printf.sprintf. The dynamic runtime doesn't
distinguish boxes/breaks, so format strings work the same as in
Printf and most Format-using OCaml programs now compile.

lib/ocaml/runtime.sx

@@ -455,6 +455,15 @@
      let printf fmt = sprintf fmt
    end ;;
 
+   module Format = struct
+     (* Thin alias of Printf for pretty-printing parity. The dynamic
+        runtime doesn't distinguish boxes/breaks — fmt strings work
+        the same as in Printf. *)
+     let sprintf fmt = Printf.sprintf fmt
+     let printf fmt = Printf.sprintf fmt
+     let asprintf fmt = Printf.sprintf fmt
+   end ;;
+
    module Lazy = struct
      let force lz = _lazy_force lz
    end ;;