PartPlate::store_to_3mf_structure read first_layer_time from the indirect cali_bboxes_data struct,
which the GUI populates at Plater.cpp:10600 but the CLI never writes to. The result was uninitialized
memory leaking into slice_info.config
Read directly from get_slice_result()->initial_layer_time, which is populated by
GCodeProcessor::finalize() in both code paths and matches the pattern already used a few lines
above for gcode_prediction.
Also default-initialize PlateBBoxData::first_layer_time to 0.0f as a defense against any other consumer
reading it without an explicit write.
