Root cause: discog_built meta timestamp was only written after ALL
of Tiers 2-4 completed. If the app was closed mid-build (context
cancelled), the timestamp was never set, so the next launch re-ran
everything from Tier 2.
Fix: track each tier independently with tier2_built, tier3_built,
tier4_built timestamps. Each tier's timestamp is written immediately
after it completes, so progress survives app restarts.
On next launch, already-completed tiers are skipped. Combined with
the incremental filterUnindexed logic, a build interrupted at
Tier 4 with 80% of similar artists done will resume from the
remaining 20%.
Also adds getLibraryArtistMBIDs helper for Tier 4 when Tier 3
was skipped (needs library MBIDs without re-running Tier 3).
InvalidateDiscographies now clears all three per-tier timestamps.
yonlu
e6692e9f1b