The home rows send popular:true genres:<slug>. The card feeder had no genre
handling (it served trending/daily), and the gateway drops a record without a
genres/<Name> key when a genres filter is present, so every genre row came back
empty.
A popular row with genres now searches subject:"<subject>" sorted by readinglog
(the same reader-shelving signal as trending, cut by genre) and stamps
genres/<Name> on each card. Up to three genres per row, deduplicated, the
trending path unchanged.
