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>

tests/playwright/generate-sx-tests.py

@@ -1881,6 +1881,14 @@ def js_expr_to_sx(expr):
     if m:
         return f'(host-get {m.group(1)} "{m.group(2)}")'
 
+    # JS keywords / literals
+    if expr in ('null', 'undefined'):
+        return 'nil'
+    if expr == 'true':
+        return 'true'
+    if expr == 'false':
+        return 'false'
+
     # Bare identifier
     if re.match(r'^[A-Za-z_]\w*$', expr):
         return expr