feat(s3/lifecycle): shard-aware meta-log reader
- ShardCount=16; ShardID(bucket,key)=top-4-bits of sha256(bucket||/||key)
- Reader subscribes via SubscribeMetadata starting at Cursor.MinTsNs(),
filters events by shard, emits to caller-owned Events channel
- Cursor: per-(shard, ActionKey) position with monotonic Advance, Freeze
for blocked actions, MinTsNs for subscription resume
- Persister interface with InMemoryPersister for tests; filer-backed
impl lands with the worker integration
