* fix(storage): keep EC .vif when deleting a coexisting regular volume
A regular volume and an EC volume for the same id share <base>.vif. When
EC shards are distributed onto a server that still holds the regular
volume — the encode source, or any replica the planner targets — the
post-encode VolumeDelete ran removeVolumeFiles and stripped the shared
.vif, leaving the freshly built EC volume without its info file.
Skip the .vif in removeVolumeFiles when an EC volume for the same id
exists on the disk (mounted, or a sealed .ecx on disk). The regular
volume's .dat/.idx still go; the EC sidecars survive.
A two-server end-to-end test encodes a volume whose source and a stub
replica both also receive shards, and asserts the final on-disk layout:
both .dat/.idx gone, each server holding only its assigned shards plus
.ecx/.vif. Storage unit tests cover the with-EC and no-EC cases, and the
Rust seaweed-volume port carries the same guard and tests.
* test(storage): assert .idx is removed in the no-EC destroy case
Strengthen TestDestroyRemovesVifWhenNoEc to confirm the full regular
volume cleanup (.dat, .idx, .vif) when no EC volume coexists.