Two dirty-mark lifecycle gaps (S19+S30):
- A commit cleared every dirty mark for the disks it scanned, even marks
recorded while the walker was already past the written prefix, so the
next dirty-subset refresh served stale cached bytes as exact until the
next full scan. Dirty marks now carry the instant they were last
recorded and a commit only clears marks that predate its scan start
(CapacityUpdate::scan_started_at, set by refresh_capacity_with_scope).
- The write side marks every disk of an EC set dirty, including remote
peers, while scans and clearing only cover local disks — remote or
removed disks stayed marked forever, keeping the dirty-disk gauge
permanently non-zero with bounded memory stuck behind it. The refresh
disk selection now prunes dirty entries outside the current local
topology before consulting them.
Ref: rustfs/backlog#1020 (S19+S30 from audit rustfs/backlog#1010)
