When background_scan() discovers zero songs in the DLC directory but the
songs table is non-empty, skip delete_missing and return with stage='error'.
An empty listing on a non-empty library almost certainly means the DLC mount
was temporarily inaccessible (FUSE remount, NTFS dirty-flag RO fallback, brief
unmount mid-scan) rather than every song being genuinely deleted.
delete_missing({}) on a non-empty DB deleted ALL rows — the #P1-libpurge
incident that wiped 50,943 songs was caused by exactly this path.
The guard fires only when current_files is empty AND the DB has at least one
row, so a genuinely empty new library is unaffected.
Test: test_scan_prune_guard.py::test_empty_listing_refuses_prune_when_db_nonempty
Failing input: dlc dir with no feedpak/sloppak/wem + 1 DB row.
Before: delete_missing({}) fires, row gone.
After: stage='error', row survives.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H2bM5jSbMskpdxm2CmuQVj
