Root cause: OnAllScansComplete called InvalidateIndexDiscographies()
which deleted the discog_built timestamp. SoftScanAllLibraries
triggers a scan whenever file counts differ (even by 1 file), so
on most launches the hook fired and wiped the cache.
The invalidation was unnecessary — Tiers 2-4 are already incremental.
filterUnindexed skips artists that are already indexed, so new
library artists with freshly-populated MBIDs get picked up naturally
without forcing a full rebuild.
InvalidateIndexDiscographies is kept as a public API for manual
rebuild (future UI button) but no longer called automatically.
Combined with per-tier timestamps from the previous commit, the
index build now correctly skips completed tiers on restart.
yonlu
91214f9d54