When PID.Artist spec changes, existing artist IDs differ from newly-computed
ones. Without remapping, all annotations (starred, ratings, play counts) become
orphaned. This change builds an artistIDMap during phase 1 and reassigns
annotations on persist, mirroring the existing albumIDMap flow.
The migration logic is gated on the previous spec being non-empty and different
from the current spec, so the default "name" spec (byte-identical to the
historical hardcoded artistID) triggers no remapping on upgrade.