lvc_

It doesn't necessarily need to be public in the global sense, but it is public in the sense of "you can't rely on keeping it secret as a security boundary". Since its used to validate the signature, it will usually need to be (at least) as public as the signed thing.

Free software doesn't need to be copyleft, which is what you're describing - eg, the FSF explicitly considers 3 clause BSD to be a free license. Similarly Open Source (as used by the OSI) doesn't require that you can incorporate into proprietary projects and it considers the GPL to be open source. Somewhat famously the definitions of the two terms are basically equivalent even though they're motivated quite differently.

Would you be willing to link your ML layout?

Not disagreeing we've moved on from it a bit in some places, but.. "Everything is a file" never meant "everything is a disk file" - think of std{in,out,err} or network sockets. The implications are more around having a standard set of syscalls for moving data in and out of a program, abstracting away where it actually lives and treating the data itself as something opaque.