A dirty-subset refresh scans only the dirty disks, so its raw
`CapacityUpdate` carries just that subset's `total_used`/`file_count`.
`update_capacity` recomputed the correct cluster-wide total into a local
variable and wrote it to the cache, but never wrote it back to the
`CapacityUpdate` that `refresh_or_join`/`spawn_refresh_if_needed` return
and publish to joiners. The admin blocking path (`resolve_admin_used_capacity`
-> `refresh_or_join_admin_disks(allow_dirty_subset=true)`) consumes
`update.total_used`, so a single dirty disk in an N-disk cluster made
admin StorageInfo report only the scanned subset's bytes (a large
transient undercount for that request and same-cycle joiners).
`CachedDiskCapacity` also dropped each disk's `file_count`/`is_estimated`,
so subset refreshes could not recompute a correct cluster file count and
would launder an estimated per-disk value into an exact cluster total.
Fix:
- `update_capacity` now reconciles `total_used`, `file_count`, and
`is_estimated` from the full per-disk cache and returns the corrected
`CapacityUpdate`.
- `CachedDiskCapacity` stores `file_count`/`is_estimated` per disk;
`file_count` sums the cache and `is_estimated` is the OR across disks.
- `refresh_or_join` and `spawn_refresh_if_needed` rebind their result to
the reconciled update so the leader return value and the value
published to joiners both carry cluster totals.
Tests: extend the subset-refresh test to assert the returned update's
reconciled `total_used`/`file_count`/`is_estimated`, and add a
`refresh_or_join` dirty-subset test asserting the leader returns the
merged cluster total (not the subset sum) and matches the cache.
Refs: https://github.com/rustfs/backlog/issues/1011
