Describe the theoretical framework underpinning quality management systems $($QMS$)$ in operational civil engineering, focusing on Total Quality Management $($TQM$)$ principles. Expound upon the fundamental concepts of TQM$,$ including continuous improvement, customer focus, and employee empowerment, and their applicability within civil engineering operations. Discuss the integration of quality planning, assurance, and control processes to ensure adherence to project specifications and standards, emphasizing the role of leadership commitment and organizational culture in fostering a quality$-$driven approach. Furthermore, analyze the theoretical underpinnings of performance measurement metrics, such as Key Performance Indicators $($KPIs$)$ and Six Sigma methodologies, in monitoring and enhancing quality performance throughout the project lifecycle, elucidating their significance in achieving excellence in civil engineering projects.