Mirror the reporter's topology — single volume server with 8 hdd
locations (/mnt/d{1..8}/weed), .ecx and shards spread across disks the
way the volume server places them after a few rebuild rounds — and
exercise three states: clean, 0-byte residue on the rebuild disk,
0-byte residue on a sibling disk. Each variant must rebuild the missing
shards byte-identical to the originals AND decode the data shards back
through WriteDatFile to a .dat that matches the original byte-for-byte,
which is the real safety check that non-zero rebuilt files aren't
masking corrupted content.