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>

spec/primitives.sx

@@ -1326,3 +1326,9 @@
   :params ((lst :as list))
   :returns "bytevector"
   :doc "Build a bytevector from a list of byte integers 0-255.")
+
+(define-primitive
+  "format"
+  :params ((template :as string) &rest args)
+  :returns "string"
+  :doc "CL-style format string. Directives: ~a display, ~s write, ~d decimal, ~x hex, ~o octal, ~b binary, ~f fixed-point, ~e scientific, ~% newline, ~& fresh-line, ~~ tilde, ~t tab. Optional first arg: output-port.")