* fix(ec): prefer credible replica as canonical metric in EC detection
An interrupted encode can leave a 0-byte .dat replica behind. When that
stub sits on a lower-sorting server than the real replica, the
lowest-server canonical pick reported Size=0, tripped the min-size gate,
and the volume was stranded in skippedTooSmall: detection never proposed
an encode, so the partial EC shards were never cleared and re-distribute
kept hitting the mounted-volume guard.
selectCanonicalMetric now prefers the lowest-server credible replica
(data-bearing, not already EC), falling back to the lowest-server metric
only when nothing is credible so the downstream gates skip as before. A
leftover EC shard set on a lower server no longer short-circuits the
volume at the IsECVolume guard either, so the orphan-source cleanup and
re-encode paths get their chance.
* fix(ec): treat a bare superblock .dat as a stub too
An interrupted encode or copy can write the 8-byte superblock and then
fail, leaving an 8-byte .dat with no data. isStubReplica used a strict <
so that file slipped through as credible, could win the canonical pick on
a low server, and re-tripped the min-size gate. Use <= the superblock so a
data-less .dat never shadows a real replica.
