* volume: copying a remote-backed volume only needs space for the index
VolumeCopy sized its target-location check by the source .dat even when
that .dat lives in a cloud tier and only .idx/.vif land locally, so
re-replicating a tiered volume demanded the full remote size in free
disk. Require the index size instead.
* shell: volume.tier.upload keeps volume replicas
Tiering a replicated volume deleted every replica but the upload
source, leaving one server holding the only .idx and the only .vif
that knows the remote object key — losing that server orphaned the
volume even though its data sat intact in the cloud.
Replicate the uploaded .idx/.vif onto the other replica servers
instead (VolumeCopy skips the .dat for remote-backed volumes), so all
replicas serve reads from the same remote object and the volume keeps
its replica count. An already-tiered replica is preferred as the
upload source, so a rerun after a partial failure reuses the existing
remote object instead of uploading a second copy under a new key.
* shell: group tier upload locations instead of re-prepending
* rust volume: copying a remote-backed volume only needs space for the index
Mirror the Go VolumeCopy change: size the free-location check by the
source .idx when the .dat lives in a cloud tier, since only .idx/.vif
land locally.
