refactor(ecstore): make store-to-disk error narrowing a named fallible operation
Backlog#1845 step 4. The blanket impl From<StorageError> for DiskError let ? silently push store-only errors (locks, buckets, quotas) across the disk boundary into DiskError::other, where the rendered message fragments reduce_errs quorum buckets. Same story for the blanket From<StorageError> for rustfs_filemeta::Error and its other() catch-all.
Both impls are replaced by named, fallible methods: StorageError::narrow_to_disk() and StorageError::narrow_to_filemeta(). Variants with an identity on the far side map across unchanged - including the two documented lossy collapses (SlowDown -> TooManyOpenFiles, StorageFull -> DiskFull) that the round-trip tests pin - and everything else returns Err(self) so the call site decides what crossing the boundary means. Removing the impls let the compiler enumerate every conversion site; the census that scoped this issue had found 5, the compiler found 33.
Call sites keep their existing behavior: the io identity bridge and the generic sites fold Err into the io-backed other() exactly as the old catch-all did (identity still recoverable by downcast), listing paths use one shared to_filemeta_err helper, and the two sites that relied on the SlowDown collapse now construct DiskError::TooManyOpenFiles directly so the loss is visible where it happens. No behavior change intended anywhere; the io::Error bridge itself is untouched by design.
Ref rustfs/backlog#1845
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
cxymds