counts.history was a durable write counter. the history retention filter destroys expired entries during compaction with no way to decrement it, so with HYDRANT_HISTORY_TTL set it only ever grew while the keyspace stayed bounded. it also counted a history key twice when a second version was replaced at the same death rev (a commit rev ahead of the local clock), which no decrement could undo. /stats now reads counts.history from the history keyspace's table and memtable metadata, the same way events and relay_events are already counted, so retention, repo erase and overwrites all show up once compaction reaches them. the counter and its per commit delta write are gone, which takes one count delta row off every commit that writes history. nothing outside hydrant reads the field, and the name stays the same. the old persisted counter key stays behind in existing databases but nothing reads it. history_count_drops_what_retention_expires and history_count_follows_overwrites_and_purges go through Hydrant::stats. against the old counter code the first reports 2 after the entries expired and were compacted (want 0) and the second reports 2 after a same key overwrite (want 1). docs/concepts/retention.md says what the number means, and HISTORY_TTL gets its missing row in docs/configuration.md. fixes hydrant-2xc and hydrant-aze.