The pre-encode cluster sweep deleted only the listed canonical shards on
remote nodes, leaving index/sidecar (and, on builds with versioned
generations, those too) behind. Add a full_teardown flag to
VolumeEcShardsDelete that evicts the volume and wipes every EC artifact for
it on every disk via removeStaleEcArtifacts; the shell and worker pre-encode
cleanup paths set it. Other delete callers (balance/decode/repair) are
unchanged.