Can you Lock the photoslibrary file so you do not accidentally trash or move it?

You can lock a Photos library

and that will give you a warning if you try to delete it:

However, you can still override the warning and delete the library. But you're warned.

Locking doesn't affect working with the library.

No idea if locking it is possible - more knowledgable others will be along if it is.

But your best protection from this sort of thing is a time machine backup. Or other suitable backup - but time machine is excellent, and easy to configure. You just need a large enough external drive. You can even set it up on some network drives (I use Synology).

A proper backup will protect you not only from damaging/deleting your library, but also (for example) from unintentionally deleting individual images. Not to mention all your other data on your new Mac.