* fix(storage): delete and unmount every copy of a duplicate volume id
NewStore has no cross-disk duplicate guard (unlike the Rust volume
server, which refuses to start in that state), so a stale twin of a
volume id can mount on a second disk after a disk repair. DeleteVolume
and UnmountVolume returned after the first matching disk, leaving the
twin to survive and re-register as the volume's content. Walk every disk
and act on all copies, emitting one heartbeat delta per copy.
* fix(storage): surface partial delete/unmount failures across duplicate copies
Address review: if removing one copy of a duplicate volume id fails with a
real error (disk IO, permissions), the loop logged it and could still
return success once another copy was removed -- leaving the stale copy to
re-register, the exact divergence this guards against. DeleteVolume and
UnmountVolume now accumulate such errors and return them (still attempting
every disk), so a copy left behind is never reported as success. Add a
DeleteVolume duplicate-copies regression test.