feat(s3/lifecycle): event router + DueTime schedule
Router consumes per-shard reader events, looks up matching ActionKeys via
the engine's BucketActionKeys index, and emits Matches with DueTime =
event_time + action.Delay. Evaluation runs at DueTime so the age gate
passes for fresh events; the dispatcher's identity-CAS catches drift.
Schedule is a min-heap by DueTime; duplicates allowed (RPC CAS handles
the redundant dispatch as NOOP_RESOLVED). BucketActionKeys accessor
added to engine.Snapshot.
