New package internal/sync. LogChange writes a library_changes row inside
the supplied tx. Encode helpers produce stable composite ids for like_*
and playlist_track entries. Subsequent commits wire LogChange into the
scanner / likes / playlists services.
Also: dbtest.dataTables now includes library_changes so test isolation
holds across runs.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
