Must I marry the girl with whom I committed zinā in order to protect her and prevent her from committing fahishah with someone else, considering my fear that she might repeat her actions with another person?

You are obligated to promptly repent to Allah for the act of adultery you committed, by abandoning the sin, regretting it, and resolving never to return to it, while also maintaining discretion. If both of you repent, then there is no objection to you marrying her after her womb has been cleared, for a sincere repentance erases what came before it. If both of you repent and you marry her, then establish Allah's limits in your home and maintain a good opinion of her. However, if her repentance is not evident to you, then do not marry her, and hasten to marry a righteous woman who will help you remain chaste.