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.

src/AcDream.Core.Net/Packets/TransportDisconnect.cs

@@ -0,0 +1,31 @@
+namespace AcDream.Core.Net.Packets;
+
+/// <summary>
+/// Builds retail's connection-level disconnect packet.
+/// </summary>
+/// <remarks>
+/// <c>ClientNet::LogOffServer</c> at <c>0x00543EF0</c> creates the
+/// <c>0x8000</c> optional header and sends it through
+/// <c>SharedNet::SendOptionalHeader</c> at <c>0x00543160</c>. That path
+/// zero-initializes the packet sequence and copies the receiver's network id
+/// and iteration into the fixed header. The disconnect is cleartext and has
+/// no body.
+/// </remarks>
+public static class TransportDisconnect
+{
+    public static byte[] Build(ushort networkId, ushort iteration)
+    {
+        var header = new PacketHeader
+        {
+            Sequence = 0,
+            Flags = PacketHeaderFlags.Disconnect,
+            Id = networkId,
+            Iteration = iteration,
+        };
+
+        return PacketCodec.Encode(
+            header,
+            ReadOnlySpan<byte>.Empty,
+            outboundIsaac: null);
+    }
+}