The parent folder's images_updated_at now counts only when the reader could
actually serve the album-root cover: multiple album folders (disc layout), or a
single folder with no images of its own. This mirrors albumRootParent's first
qualification gate, so an unrelated artist-level image change no longer advances
(and temporarily suppresses) the version of every sibling album with its own
cover. The remaining gates (other-audio, library root) stay unmirrored: with
omission plus the write-side clamp, residual over-approximation only causes a
short suppression that closes on the next serve. Plan verified on a 96K-track
production copy: unchanged, all PK point lookups.
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