spec: format — CL-style string formatting (~a ~s ~d ~x ~o ~b ~f ~% ~& ~~ ~t)

28 tests, passes on both JS and OCaml.
- spec/stdlib.sx: pure SX format function
- spec/primitives.sx: format primitive declaration
- lib/r7rs.sx: fix number->string to support optional radix arg
- hosts/ocaml: add format-decimal primitive, load stdlib.sx in test runner
- hosts/javascript: load stdlib.sx in test runner

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

hosts/ocaml/lib/sx_primitives.ml

@@ -2806,6 +2806,13 @@ let () =
     match args with
     | [v] -> String (sx_display_val v)
     | _ -> raise (Eval_error "display-to-string: 1 arg"));
+  register "format-decimal" (fun args ->
+    match args with
+    | [Integer n; Integer prec] -> String (Printf.sprintf "%.*f" prec (float_of_int n))
+    | [Number n;  Integer prec] -> String (Printf.sprintf "%.*f" prec n)
+    | [Integer n; _] -> String (Printf.sprintf "%.6f" (float_of_int n))
+    | [Number n;  _] -> String (Printf.sprintf "%.6f" n)
+    | _ -> raise (Eval_error "format-decimal: expected number precision"));
   register "current-input-port"  (fun _ -> Nil);
   register "current-output-port" (fun _ -> Nil);
   register "current-error-port"  (fun _ -> Nil);