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/tests/Fleet.Tests/TestTables.cs

@@ -0,0 +1,13 @@
+using Fleet.Rpc;
+
+namespace Fleet.Tests;
+
+/// <summary>测试里用的方法声明：除非另有说明，方法都由拨号方发给监听方。</summary>
+internal static class TestTables
+{
+    public static MethodTable Of(params MethodSpec[] specs) => new(specs);
+
+    public static MethodSpec Req(string name, RpcSide from = RpcSide.Dialer) => MethodSpec.Request(name, from);
+
+    public static MethodSpec Note(string name, RpcSide from = RpcSide.Dialer) => MethodSpec.Notification(name, from);
+}