js-on-sx: scope var defines + js-args for call args

JS top-level var was emitting (define <name> X) at SX top level,
permanently rebinding any SX primitive of that name (e.g. var list
= X broke (list ...) globally). Two-part fix:
1. wrap transpiled program in (let () ...) in js-eval so defines
   scope to the eval and don't leak.
2. rename call-args constructor in js-transpile-args from list to
   js-args (a variadic alias) so even within the eval's own scope,
   JS vars named list don't shadow arg construction.
Array-literal transpile keeps list (arrays must be mutable).
built-ins/Object: 41/50 → 42/50. conformance.sh: 148/148.

lib/js/transpile.sx

@@ -426,7 +426,7 @@
               (list (js-sym "list") "js-spread" (js-transpile (nth e 1)))
               (list (js-sym "list") "js-value" (js-transpile e))))
           args))
-      (cons (js-sym "list") (map js-transpile args)))))
+      (cons (js-sym "js-args") (map js-transpile args)))))
 
 ;; Transpile a JS expression string to SX source text (for inspection
 ;; in tests). Useful for asserting the exact emitted tree.
@@ -1451,7 +1451,7 @@
   (fn
     (src)
     (let
-      ((result (eval-expr (js-transpile (js-parse (js-tokenize src))))))
+      ((result (eval-expr (list (quote let) (list) (js-transpile (js-parse (js-tokenize src)))))))
       (js-drain-microtasks!)
       result)))