spec: rational numbers — 1/3 literals, arithmetic, numeric tower integration

SxRational type in OCaml (Rational of int * int, stored reduced, denom>0)
and JS (SxRational class with _rational marker). n/d reader syntax in
spec/parser.sx. Arithmetic contagion: int op rational → rational, rational
op float → float. JS keeps int/int → float for CSS backward compatibility.
OCaml as_number + safe_eq extended for cross-type rational equality so
(= 2.5 5/2) → true. 62 tests in test-rationals.sx, all pass.
JS: 2232 passed. OCaml: 4532 passed (+11 vs pre-fix baseline).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -1394,6 +1394,7 @@ let rec dispatch env cmd =
         | Char n -> Sx_types.inspect (Char n)
         | Eof -> Sx_types.inspect Eof
         | Port _ -> Sx_types.inspect result
+        | Rational (n, d) -> Printf.sprintf "%d/%d" n d
         | _ -> "nil"
       in
       send_ok_raw (raw_serialize result)