prune_never_active_keyed_sessions now returns (deleted, entries_dropped,
skipped) from its own guarded-row list. `sessions prune --never-active`
printed candidates-minus-deleted, which lumped rows that vanished
concurrently in with live-turn skips and needed an "or already gone"
hedge; it now reports only rows spared by a live turn or compression lock.
The docstring claimed a failed sweep rolls back whole; the row sweep is one
transaction but routing entries are dropped in a second write after it
commits, so say that.
The kept guard test now also covers a candidate that vanishes between
listing and the sweep: its routing entry must still be dropped (the r1
per-row loop left it behind, so the gateway would resume a nonexistent id).
13cf8f2058