feat(net): graceful CharacterLogOff + DISCONNECT on session Dispose

When closing the acdream window, WorldSession.Dispose now sends
CharacterLogOff (game message 0xF653, no payload) followed by a bare
DISCONNECT control packet (header flag 0x8000) before closing the
UDP socket. This tells ACE to release the character lock immediately
instead of waiting for the ~60s network timeout — which was blocking
rapid iteration during testing since acdream now does a proper login
(Phase 4.8-4.10) and ACE holds the character in-world.

Pattern from references/holtburger/.../client/commands.rs lines
879-892 (Quit handler). Best-effort: if the socket is already dead,
the exception is eaten and Dispose finishes cleanup normally.

220 tests still green.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core.Net/WorldSession.cs

@@ -431,5 +431,45 @@ public sealed class WorldSession : IDisposable
         StateChanged?.Invoke(next);
     }
 
-    public void Dispose() => _net.Dispose();
+    /// <summary>
+    /// Graceful shutdown: tell the server we're leaving so it releases the
+    /// character lock immediately instead of waiting 60s for the session to
+    /// time out. Pattern from
+    /// <c>references/holtburger/crates/holtburger-core/src/client/commands.rs</c>
+    /// lines 879-892: send <c>CharacterLogOff</c> game message (opcode
+    /// 0xF653, no payload) then send a bare <c>DISCONNECT</c> control
+    /// packet (header flag 0x8000, no payload).
+    /// </summary>
+    public void Dispose()
+    {
+        if (CurrentState == State.InWorld)
+        {
+            try
+            {
+                // Tell ACE "player is leaving the world" so it cleans up
+                // the character immediately.
+                var logoff = new Packets.PacketWriter(8);
+                logoff.WriteUInt32(0xF653u);  // CharacterLogOff opcode
+                SendGameMessage(logoff.ToArray());
+
+                // Tell the transport layer "close this session."
+                var disconnectHeader = new PacketHeader
+                {
+                    Sequence = _clientPacketSequence++,
+                    Flags    = PacketHeaderFlags.Disconnect,
+                    Id       = _sessionClientId,
+                };
+                byte[] disconnectPacket = PacketCodec.Encode(
+                    disconnectHeader, ReadOnlySpan<byte>.Empty, outboundIsaac: null);
+                _net.Send(disconnectPacket);
+            }
+            catch
+            {
+                // Best-effort — if the socket is already dead, eat the
+                // exception and let Dispose finish cleaning up.
+            }
+        }
+
+        _net.Dispose();
+    }
 }