sx: format_number helper — defuse int_of_float overflow on huge floats

Shared formatter in sx_types.ml. Small integer-valued floats still print
as plain ints; floats outside safe-int range (|n| >= 1e16) now print as
%.17g (full precision) instead of silently wrapping to negative or 0.
Non-integer values keep %g 6-digit behavior — no existing SX tests regress.

Unblocks Number.MAX_VALUE / Math.pow(2,N) style tests in js-on-sx where
iterative float loops were collapsing to 0 at ~2^63.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_runtime.ml

@@ -15,9 +15,7 @@ let prim_call name args =
 (** Convert any SX value to an OCaml string (internal). *)
 let value_to_str = function
   | String s -> s
-  | Number n ->
-    if Float.is_integer n then string_of_int (int_of_float n)
-    else Printf.sprintf "%g" n
+  | Number n -> Sx_types.format_number n
   | Bool true -> "true"
   | Bool false -> "false"
   | Nil -> ""