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/bin/sx_server.ml

@@ -165,9 +165,7 @@ let rec serialize_value = function
   | Nil -> "nil"
   | Bool true -> "true"
   | Bool false -> "false"
-  | 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
   | String s -> "\"" ^ escape_sx_string s ^ "\""
   | Symbol s -> s
   | Keyword k -> ":" ^ k
@@ -214,9 +212,7 @@ let rec serialize_value_hashed (index : (string, string) Hashtbl.t) = function
   | Nil -> "nil"
   | Bool true -> "true"
   | Bool false -> "false"
-  | 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
   | String s -> "\"" ^ escape_sx_string s ^ "\""
   | Symbol s when String.length s > 0 && s.[0] = '~' ->
     (match Hashtbl.find_opt index s with
@@ -1318,7 +1314,7 @@ let rec dispatch env cmd =
       let rec raw_serialize = function
         | Nil -> "nil"
         | Bool true -> "true" | Bool false -> "false"
-        | 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
         | String s -> "\"" ^ escape_sx_string s ^ "\""
         | Symbol s -> s | Keyword k -> ":" ^ k
         | List items | ListRef { contents = items } -> "(" ^ String.concat " " (List.map raw_serialize items) ^ ")"
@@ -1364,9 +1360,7 @@ let rec dispatch env cmd =
         | Nil -> "nil"
         | Bool true -> "true"
         | Bool false -> "false"
-        | 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
         | String s -> "\"" ^ escape_sx_string s ^ "\""
         | Symbol s -> s
         | Keyword k -> ":" ^ k
@@ -3192,7 +3186,7 @@ let pp_atom = Sx_types.inspect
 
 let rec est_width = function
   | Nil -> 3 | Bool true -> 4 | Bool false -> 5
-  | Number n -> String.length (if Float.is_integer n then string_of_int (int_of_float n) else Printf.sprintf "%g" n)
+  | Number n -> String.length (Sx_types.format_number n)
   | String s -> String.length s + 2
   | Symbol s -> String.length s
   | Keyword k -> String.length k + 1