broker: clear file_prefix for the calibration sequence, so status no longer shows the previous run's prefix

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PXc1zD1yxHqaR1DGdCTHL4

broker/JFJochStateMachine.cpp

@@ -85,6 +85,8 @@ bool JFJochStateMachine::CalibrateJUNGFRAU(std::unique_lock<std::mutex> &ul) {
 
 void JFJochStateMachine::CalibrateDetector(std::unique_lock<std::mutex> ul) {
     cancel_sequence = false;
+    // Status and messages must not show the previous run's prefix; calibration writes no files anyway
+    experiment.FilePrefix("");
 
     pixel_mask = PixelMask(experiment);
     UpdatePixelMaskStatistics(pixel_mask.GetStatistics());