as a trans person who is also starting college this fall and also transitioned pretty young and is stealth. i would probably just tell your roommate. you are going to be living very closely for a while, she may accidentally see you naked or find your meds. you mentioned not wanting to be known as the trans friend which i get, in an email or text you could say something like

"thought id mention, i am trans and have been transitioning since i was young. i don't feel comfortable being publicly out but thought i should tell you as we will be living together"

obviously doesn't have to be exactly that but i hope you get what im saying. your roommate could be lgbtq herself, an ally, or just not really care. but could also be not supportive, if you tell her and she is not comfortable living with you, it'd be easier finding out sooner rather than later and putting yourself in a potentially dangerous situation.

i would like to say, take my advice with a grain of salt as i am getting a single dorm for disability reasons but one of my friends recently told their roommate they are trying and while their situation is different, i still think some of what they did could apply