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/IdentityAndRpcTests.cs

@@ -67,7 +67,7 @@ public sealed class IdentityAndRpcTests
     [Fact]
     public async Task Router_DispatchesTypedRequests_AndReportsStructuredErrors()
     {
-        var router = new RpcRouter<string>()
+        var router = new RpcRouter<string>(RpcSide.Listener, TestTables.Of(TestTables.Req("test.add"), TestTables.Req("test.boom"), TestTables.Req("test.nope")))
             .On("test.add", EchoContext.Default.AddParams, EchoContext.Default.AddResult,
                 (caller, p, _) => ValueTask.FromResult(p.A < 0 ? Result<AddResult>.Fail(FleetErrors.Forbidden, "negative") : Result<AddResult>.Ok(new AddResult { Sum = p.A + p.B, Caller = caller })))
             .On<AddParams, AddResult>("test.boom", EchoContext.Default.AddParams, EchoContext.Default.AddResult, (_, _, _) => throw new InvalidOperationException("secret detail"));
@@ -92,7 +92,7 @@ public sealed class IdentityAndRpcTests
     public async Task Router_HandlerExceptions_AreContained_AndNeverLeakDetails()
     {
         string? logged = null;
-        var router = new RpcRouter<string>((method, ex) => logged = method + ":" + ex.Message)
+        var router = new RpcRouter<string>(RpcSide.Listener, TestTables.Of(TestTables.Req("test.boom")), onUnhandled: (method, ex) => logged = method + ":" + ex.Message)
             .On<AddParams, AddResult>("test.boom", EchoContext.Default.AddParams, EchoContext.Default.AddResult, (_, _, _) => throw new InvalidOperationException("secret detail"));
 
         RpcMessage reply = (await router.DispatchAsync("a", Request(1, "test.boom", """{"a":1,"b":1}"""), CancellationToken.None))!;
@@ -105,7 +105,7 @@ public sealed class IdentityAndRpcTests
     public async Task Router_Notifications_ProduceNoResponse_EvenOnFailure()
     {
         int calls = 0;
-        var router = new RpcRouter<string>().OnNotify("test.note", EchoContext.Default.AddParams, (_, _, _) =>
+        var router = new RpcRouter<string>(RpcSide.Listener, TestTables.Of(TestTables.Note("test.note"), TestTables.Note("test.unknown"))).OnNotify("test.note", EchoContext.Default.AddParams, (_, _, _) =>
         {
             calls++;
             return ValueTask.CompletedTask;