SEP 53: Metadata switch uid to key

Currently the unique identifier for a document or other object is stored in its metadata as “uid”.

Key seems perhaps more in line with my desire for metadata keys to be clear and descriptive than the somewhat outwardly opaque “uid”, it also may fit better grammatically as the iteration toward a more database-like schema continues.

This is a non-essential, non-urgent proposal with no meaningful ramifications, parked here so I can continue to weight it up.