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/bin/run_tests.ml

@@ -2872,6 +2872,7 @@ let run_spec_tests env test_files =
       match sx_vm_execute with
       | Some fn -> Sx_ref.cek_call fn (List args)
       | None -> Nil)));
+  load_module "stdlib.sx" spec_dir;   (* pure SX stdlib: format etc. *)
   load_module "signals.sx" spec_dir;  (* core reactive primitives *)
   load_module "signals.sx" web_dir;   (* web extensions *)
   load_module "freeze.sx" lib_dir;

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);