fix(net): honor retail graceful logout handshake

Send the active character id, drain until the authoritative server confirmation, then emit retail's zero-sequence connection disconnect with the negotiated receiver iteration. The connected gate now waits for ACE to remove the exact UDP session before reconnecting, eliminating fixed-delay races.

tests/AcDream.Core.Net.Tests/Packets/TransportDisconnectTests.cs

@@ -0,0 +1,26 @@
+using AcDream.Core.Net.Packets;
+
+namespace AcDream.Core.Net.Tests.Packets;
+
+public sealed class TransportDisconnectTests
+{
+    [Fact]
+    public void Build_MatchesRetailOptionalHeaderShape()
+    {
+        byte[] datagram = TransportDisconnect.Build(0x1234, 0x0001);
+
+        Assert.Equal(PacketHeader.Size, datagram.Length);
+
+        PacketHeader header = PacketHeader.Unpack(datagram);
+        Assert.Equal(0u, header.Sequence);
+        Assert.Equal(PacketHeaderFlags.Disconnect, header.Flags);
+        Assert.Equal((ushort)0x1234, header.Id);
+        Assert.Equal((ushort)0x0001, header.Iteration);
+        Assert.Equal((ushort)0, header.DataSize);
+
+        PacketCodec.PacketDecodeResult decoded = PacketCodec.TryDecode(
+            datagram,
+            inboundIsaac: null);
+        Assert.True(decoded.IsOk);
+    }
+}