What is the meaning of the term "قتل صبراً" (qatl ṣabran - killing while bound/imprisoned)? And is its use limited to the companions of the Prophet, peace be upon him, or does it also include idolaters, especially since Al-Nadr ibn Al-Harith was killed "ṣabran" during the Battle of Badr?

"Qatl al-sabur" (killing by sabr) means killing a living soul by pelting it with something until death. Anyone killed outside of battle, war, or by accident is considered "qatil sabur." This description refers to the method of killing, not a praise for the one killed. The Prophet, peace and blessings be upon him, said: "No Qurayshi will be killed by sabr after this day until the Day of Resurrection." This means that all of Quraysh will embrace Islam. Al-Nadr ibn al-Harith was killed by sabr on the day of Badr, and similarly, the Prophet commanded the killing of Ibn Khatal on the day of the conquest of Mecca, even though he was clinging to the curtains of the Kaaba.