Moving the library into a folder with another library loses favourites and tags #798

What happens

In Settings, changing the StemData location to a folder that already holds another StemDeck library skips the registry.json of the library being moved. The songs from the old library arrive as orphans: favourites, Trash state and tags are lost.

Who it affects

Anyone who moves their library into a folder that already contains a StemDeck library, for example one restored from a backup or shared by another machine. Nothing is deleted from disk, but the metadata a person set by hand is gone from the app.

What should happen

One of these, to be decided:

• Refuse the move and say the target already holds a library.
• Merge the two registries, and say what was merged.

Constraints

• A half-finished move can be retried, and that path already works.
• The merge needs a rule for the same song present in both registries.

Found by

Tests for the library move. The case is written as a strict expected-failure test in tests/test_stems_location.py, so it turns red as soon as the behaviour is fixed and the marker should be removed.