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;