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/hs-run-filtered.js

@@ -399,6 +399,8 @@ globalThis.cancelAnimationFrame=()=>{};
 // cluster-36b: globalFunction mock for "can call functions" test.
 // The test calls globalFunction("foo") via hyperscript and checks window.calledWith.
 globalThis.globalFunction = function(x) { globalThis.calledWith = x; };
+// asyncCheck: async-when test needs a truthy-returning global (simulates async guard).
+globalThis.asyncCheck = function() { return true; };
 // cluster-asyncError: function that returns a rejected promise.
 globalThis.failAsync = function() { return Promise.reject(new Error("boom")); };
 // HsMutationObserver — cluster-32 mutation mock. Maintains a global