cl-runtime: add lib/common-lisp/runtime.sx + test.sh (68/68 pass)

Type predicates, arithmetic, chars (inline α/digit/case),
format, gensym, values, sets, radix, list utilities.
cl-empty? guards all list traversal against () vs nil in sx_server.
Load spec/stdlib.sx in test.sh to expose format.
Fix lib/r7rs.sx number->string to use (= (len r) 0) not (nil? r).

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

lib/r7rs.sx

@@ -76,7 +76,7 @@
 (define number->string
   (let ((prim-n->s number->string))
     (fn (n &rest r)
-      (if (nil? r) (str n) (prim-n->s n (first r))))))
+      (if (= (len r) 0) (str n) (prim-n->s n (first r))))))
 
 (define
   string->number