feat: declare every protocol method's direction and enforce it in the router; add the zero-trust design [CI]

Methods, roles and binary channels are declared in one table per plane (node and peer). RpcRouter checks direction, kind and role when handlers are registered, when messages arrive (fleet_protocol_violation, disconnect after repeated violations) and before sending. NodeConnection is split by responsibility.

backend/fleet/src/Fleet.Server.Link/FleetLog.cs

@@ -19,6 +19,9 @@ internal static partial class FleetLog
     [LoggerMessage(Level = LogLevel.Warning, Message = "state push failed node={Node} type={Type} err={Err}")]
     public static partial void StatePushFailed(ILogger logger, string node, string type, string err);
 
+    [LoggerMessage(Level = LogLevel.Warning, Message = "protocol violation node={Node} method={Method} reason={Reason}")]
+    public static partial void ProtocolViolation(ILogger logger, string node, string method, string reason);
+
     [LoggerMessage(Level = LogLevel.Error, Message = "unhandled error in handler method={Method}")]
     public static partial void HandlerFailed(ILogger logger, Exception exception, string method);