ClusterClient Sender refs are temporary proxies — valid for immediate reply
but not durable for future Tells. Events now flow as DebugStreamEvent through
SiteCommunicationActor → ClusterClient → CentralCommunicationActor → bridge
actor (same pattern as health reports). Also fix DebugStreamHub to use
IHubContext for long-lived callbacks instead of transient hub instance.
The debug view polled every 2s by re-subscribing for full snapshots. Now a
persistent DebugStreamBridgeActor on central subscribes once and receives
incremental Akka stream events from the site, forwarding them to the Blazor
component via callbacks and to the CLI via a new SignalR hub at
/hubs/debug-stream. Adds `debug stream` CLI command with auto-reconnect.