VolumeTierMoveDatFromRemote downloads the .dat, trims the .vif, and swaps
the data backend to the local file, but left hasRemoteFile set. The
volume.tier.download command masks this by unmounting and remounting
right after, which reloads the flag from the trimmed .vif — but in the
window before the remount the in-memory flag is wrong: doDeleteRequest
would skip appending the tombstone to the freshly local .dat, and the
phantom-.dat guard stays disabled.
Give SwapDataBackend a hasRemoteFile argument so the backend swap and the
flag move together under one lock, and route both tier directions through
it: the tier-down download passes false, LoadRemoteFile passes true. The
flag can no longer disagree with the live backend.