Closed

The connection has terminally torn down. reason carries the raw transport close reason with the SAME semantics the old closedConnections map's String? value had: null for a graceful/local close, a non-null failure reason (e.g. receive:54, connection failed) otherwise. Terminal, monotone and dominant — see the interface KDoc. NwSeam tears a still-tracked connection IMMEDIATELY on Closed, with no grace timer.