Backlog#1845 step 1 (pure tests, no behavior change): pin the current behavior of every conversion seam an error crosses before heal, replication, or quorum aggregation classifies it, so the later typed-variant and narrow_to_disk() refactors change these expectations deliberately rather than silently.
Covered seams: DiskError <-> StorageError, DiskError <-> node_service wire Error, DiskError/StorageError <-> io::Error (the by-design identity bridge), and StorageError <-> rustfs_filemeta::Error.
Documented lossy edges pinned as-is: SlowDown collapses to TooManyOpenFiles across the disk boundary (StorageFull to DiskFull likewise), the wire Io catch-all re-wraps the rendered message on every hop and drops the io::ErrorKind, and other(format!) messages with per-disk detail fragment reduce_errs quorum buckets while identical messages still bucket together.
Ref rustfs/backlog#1845
Zhengchao An
37a50f1e5d