ReconnectReason
Why a peer's link is currently down and a reconnect / grace window is in progress.
Attached to MembershipEvent.Partitioned. A consumer driving a "reconnecting…" banner uses this to distinguish a silent Wi-Fi drop (LinkTimeout) from a peer that stopped reading (Backpressure) from a clean transport close (TransportClosed).
Session-level counterpart of PartitionEvent.Reason: the joiner-side MembershipEvent.Partitioned (host-tear) does not originate from a PartitionEvent, so the public session vocabulary owns its own type rather than leaking the lower-level liveness enum.