refactor(app): one idle-loop wake predicate for background results

hasPendingRPCResults() becomes hasPendingBackgroundResults(): the one
check the idle loop makes, every <= 200 ms, for work that finished on a
background thread and still has to be applied by update(). Lanes added
later put their term there. It still covers only the two RPC worker
lanes; no behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app.cpp

@@ -7432,7 +7432,7 @@ void App::verifyDebugCredential(const std::string& secret, std::function<void(bo
     cb(true);
 }
 
-bool App::hasPendingRPCResults() const {
+bool App::hasPendingBackgroundResults() const {
     return (worker_ && worker_->hasPendingResults())
         || (fast_worker_ && fast_worker_->hasPendingResults());
 }