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.Abstractions/FleetServerOptions.cs

@@ -12,4 +12,7 @@ public sealed class FleetServerOptions
     public TimeSpan HeartbeatTimeout { get; set; } = TimeSpan.FromSeconds(45);
 
     public TimeSpan RequestTimeout { get; set; } = TimeSpan.FromSeconds(15);
+
+    /// <summary>节点违反协议（方法没有声明、方向、类型或角色不对）的次数上限；超过就断开这条连接。</summary>
+    public int MaxViolations { get; set; } = 20;
 }