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_primitives.ml

@@ -79,9 +79,7 @@ let as_bool = function
 
 let rec to_string = 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 -> ""

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 -> ""

hosts/ocaml/lib/sx_types.ml

@@ -378,9 +378,21 @@ let env_merge base overlay =
 
 (** {1 Value extraction helpers} *)
 
+(** Format a float safely — defuse [int_of_float] overflow on huge
+    integer-valued floats, keep [%g] for fractions (unchanged). *)
+let format_number n =
+  if Float.is_nan n then "nan"
+  else if n = Float.infinity then "inf"
+  else if n = Float.neg_infinity then "-inf"
+  else if Float.is_integer n && Float.abs n < 1e16 then
+    string_of_int (int_of_float n)
+  else if Float.is_integer n then
+    Printf.sprintf "%.17g" n
+  else Printf.sprintf "%g" n
+
 let value_to_string = function
   | String s -> s | Symbol s -> s | Keyword k -> k
-  | Number n -> if Float.is_integer n then string_of_int (int_of_float n) else Printf.sprintf "%g" n
+  | Number n -> format_number n
   | Bool true -> "true" | Bool false -> "false"
   | Nil -> "" | _ -> "<value>"
 
@@ -765,9 +777,7 @@ let rec inspect = function
   | Nil -> "nil"
   | Bool true -> "true"
   | Bool false -> "false"
-  | Number n ->
-    if Float.is_integer n then Printf.sprintf "%d" (int_of_float n)
-    else Printf.sprintf "%g" n
+  | Number n -> format_number n
   | String s ->
     let buf = Buffer.create (String.length s + 2) in
     Buffer.add_char buf '"';