Get wraps an eligible original-size serve (album/artist/playlist) in a teeCachedStream
so the bytes streamed to the client are also captured; on a fully-consumed Close the
runner hashes exactly what was served. GetOrPlaceholder routes a vanished album/artist
cover through EnqueueClearIfGone, since no bytes flow through the tee on ErrUnavailable.
capAtNow keeps a future mtime out of the stored version. The mtime-preserved cover-swap
characterization test (previously pending) now passes, and the disappearing-cover e2e
serves through GetOrPlaceholder to match the real Jellyfin/Subsonic path.
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