comparePGSLibraries

Compares two libraries' model identity -- the ordered position -> (name, build) map -- without touching variant data. Two libraries with the same fingerprint hold the exact same models in the exact same order, so a result computed against one is safe to use against the other. Aborts unless both arguments are libraries.

A character scalar, one of: "identical" — The fingerprints match.

"prefix.extension" — b is strictly longer than a, and b's first length(a) (name, build) pairs equal a's -- b is a after an append-only growth, so every result computed against a stays valid against b.

"subset" — The mirror: a is b after such a growth.

"different" — Neither prefix relation holds -- a different library, a reordering, or a build change.