Ks gains inherent get/insert/remove/contains_key mirroring Keyspace's signatures with the fatal poison check built in; inherent methods win over the deref bridge, so every db.<ks>.<op>() callsite is covered without churn. the dead async insert/remove/contains_key helpers on Db are gone; Db::get checks poison before erasing the error. db/filter.rs loses its private SEP copy in favor of keys::SEP (v9 migration updated to match). part of hydrant-9xa (phase 2)