HS: fix null→nil in generator + asyncCheck fixture (+2 tests)

js_expr_to_sx bare-identifier path returned JS "null"/"undefined" as
literal symbols; added keyword mapping before the identifier regex.
Registered asyncCheck() global (returns true) for async-when test.
Regen'd spec file to propagate the null fix.

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

spec/tests/test-hyperscript-behavioral.sx

@@ -1172,7 +1172,7 @@
     ))
   (deftest "can call global javascript functions"
     (hs-cleanup!)
-    (host-set! (host-global "window") "calledWith" null)
+    (host-set! (host-global "window") "calledWith" nil)
     (let ((_el-div (dom-create-element "div")))
       (dom-set-attr _el-div "_" "on click call globalFunction(\"foo\")")
       (dom-append (dom-body) _el-div)