Promoted Bitcask compaction helpers injected into Ferricstore.Store.Shard.
Promoted hashes/sets/zsets keep large collection entries in dedicated Bitcask logs. These helpers compact those dedicated logs, preserve live compound entries, and remove obsolete/tombstone-only files when safe.
Performance boundary
Compaction is cold/control-plane compared with GET/SET/Flow writes, but it runs inside shard state and touches keydir metadata. Keep correctness first: do not add request-path calls here, and keep compaction scheduling separate from per-command write latency.