JFJochReceiverService::Start: the receiver goes first when the start fails

If std::async throws after the receiver is built, the catch reset the puller while the receiver's threads
were still polling it. It can only happen on thread or resource exhaustion.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HVeDQsU5a8AJUdJPgWevAK

receiver/JFJochReceiverService.cpp

@@ -193,6 +193,7 @@ void JFJochReceiverService::Start(const DiffractionExperiment &experiment,
         measurement = std::async(std::launch::async, &JFJochReceiverService::FinalizeMeasurement, this);
         state = ReceiverState::Running;
     } catch (const std::exception &e) {
+        receiver.reset(); // its threads may be polling the puller reset below
         // The receiver never started, so drop the status its base constructor had already reset -
         // otherwise /status and /statistics keep reporting a zero-progress run that never happened,
         // until the next start overwrites it.