The local spill backend never reclaimed its files, so configured roots
grew without bound and default per-process dsh-spill-* temp roots piled
up across runs. Immediate deletion is unsafe because persisted, resumed,
and forked sessions may still reference an older locator.
Add a fiber-owned, best-effort sweep that runs once after activation
(never delaying availability, awaited on disposal): it deletes regular
files older than cleanupPeriodDays (default 30; 0 disables) across the
configured root and prior default temp roots, prunes emptied dirs, and
skips symlinks/unknown entries. Every filesystem failure is contained
and logged, so the sweep cannot fail activation or a concurrent write.
