- CacheWorker no longer purges the files of a download that is still in-flight
or downloaded and awaiting install, so missing the system install prompt for
longer than the cache window no longer forces a re-download.
- observeDownloads now starts the next queued download only once nothing else
is purchasing/downloading/verifying, instead of merely checking for a
DOWNLOADING row. This serializes downloads so concurrent workers can't clobber
the shared foreground/progress notification.
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