"Necromancy" means different things to different people, and the term has been used in fantasy literature many distinctly different ways. Some talk to ghosts, some curse the bodies of others, some raise zombies - hey, I don't judge, it's good to have a hobby. So, the solution is easy - it must be defined for the mage, and that definition must be in step with the limits of the Virtue.
But what, exactly, it does encompass is flexible within the limits of "smaller than a single Art" or "slightly narrower than a single Tech/Form combo", depending whether it's the Major or Minor focus.
!!BUZZER!! - Oh, sorry, no, but thanks for playing.
At least, if you're getting this from HoH:TL, I think you should reread. If somewhere else, sorry, and then I need to read that.
First, while TL does refer to a necromantic tendency within the House, that book doesn't state that Tremere necromancers are also healers - it says that Tremere physicians tend to also be corporeal necromancers. Big diff. And it doesn't imply the Focus covers both.
It also repeatedly uses the term "corporeal necromancers", which pretty clearly covers Corpus but not Mentem, answering your question.
(If you're getting this from elsewhere, again, lemme know the source.)