Chunked

Persist a high-water mark every chunk events. On a cold start the counter skips to the next chunk boundary: duplicates are impossible, but a crash can introduce an artificial gap of at most chunk. Prefer EveryEvent unless events are extremely frequent.

Session activity is persisted at each chunk boundary and whenever the app is backgrounded, so a session that ends with a graceful background always restores correctly. A hard crash between boundaries can leave the persisted activity time up to one chunk's worth of events stale — negligible against the session timeout at the high event rates this mode is meant for. Use EveryEvent if precise session timing must survive a hard crash mid-session.