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Group cells in the Readest Library view rendered as FakeCover even when their child books had perfectly good cloud covers — the queue that fetches <hash>.png covers was only primed for cloud-only book entries on the visible page, so children of group entries were never requested. A later partial composite (3/4 covers) was also written to a content-fingerprinted disk cache and kept serving forever, since the fingerprint stayed the same after the 4th cover landed. Two fixes wired together: - libraryitem.set_visible_hashes now expands visible group entries to include their first-N children's hashes, so trigger_download's visibility filter no longer rejects them. - group_covers.child_cover_bb queues a cloud-cover download when the fallback path misses for a cloud-present book. Capped at 4 per group by the existing cells_for(shape) limit. Disk caching of composites is dropped entirely; mosaics are recomposed in memory each paint. New spec/library/group_covers_spec.lua locks the contract for URI round-trip, child_cover_bb's missing-cover branches, and libraryitem's group-children expansion. Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>