refactor(channelserver): remove Channels fallbacks, use Registry as sole cross-channel API

main.go always sets both Channels and Registry together, making the
Channels fallback paths dead code. This removes:

- Server.Channels field from the Server struct
- 3 if/else fallback blocks in handlers_session.go (replaced with
  Registry.FindChannelForStage, SearchSessions, SearchStages)
- 1 if/else fallback block in handlers_guild_ops.go (replaced with
  Registry.NotifyMailToCharID)
- 3 method fallbacks in sys_channel_server.go (WorldcastMHF,
  FindSessionByCharID, DisconnectUser now delegate directly)

Updates anti-patterns.md #6 to "accepted design" — Session struct is
appropriate for this game server's handler pattern, and cross-channel
coupling is now fully routed through the ChannelRegistry interface.

server/channelserver/sys_session_test.go

@@ -55,17 +55,19 @@ func createTestSession(mock network.Conn) *Session {
 	// Create a production logger for testing (will output to stderr)
 	logger, _ := zap.NewProduction()
 
+	server := &Server{
+		erupeConfig: &cfg.Config{
+			DebugOptions: cfg.DebugOptions{
+				LogOutboundMessages: false,
+			},
+		},
+	}
+	server.Registry = NewLocalChannelRegistry([]*Server{server})
 	s := &Session{
 		logger:      logger,
 		sendPackets: make(chan packet, 20),
 		cryptConn:   mock,
-		server: &Server{
-			erupeConfig: &cfg.Config{
-				DebugOptions: cfg.DebugOptions{
-					LogOutboundMessages: false,
-				},
-			},
-		},
+		server:      server,
 	}
 	return s
 }