The previous backfill called rerankArtists with an incomplete pop
map (only backfilled artists), wiping out scores for artists that
had index data (including the library-boosted Shannon and the Clams).
Now backfill only updates Score for artists that were actually
backfilled from LB, using OriginalScore as the relevance input
and a maxPop computed across both index and backfill data. Artists
with existing index scores are untouched. A final sort by Score
merges both groups into the correct order.
yonlu
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