js-on-sx: global eval(src) actually evaluates the source

Was returning the input unchanged: eval('1+2') gave "1+2".
Per spec, eval(string) parses and evaluates as JS; non-string
passes through. Wired through js-eval (existing
lex/parse/transpile/eval pipeline).
built-ins/String fail count 13 → 11. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -37,7 +37,11 @@
 ;; per JS (technically ToNumber("") === 0).
 (define
   js-global-eval
-  (fn (&rest args) (if (empty? args) :js-undefined (nth args 0))))
+  (fn (&rest args)
+    (cond
+      ((empty? args) :js-undefined)
+      ((not (= (type-of (nth args 0)) "string")) (nth args 0))
+      (else (js-eval (nth args 0))))))
 
 ;; Safe number-parser. Tries to call an SX primitive that can parse
 ;; strings to numbers; on failure returns 0 (stand-in for NaN so the