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/handlers_session_test.go

@@ -281,7 +281,7 @@ func TestHandleMsgSysLockGlobalSema_RemoteMatch(t *testing.T) {
 		clients:             make(map[*Session]uint32),
 		reservedClientSlots: make(map[uint32]bool),
 	})
-	server.Channels = []*Server{server, otherChannel}
+	server.Registry = NewLocalChannelRegistry([]*Server{server, otherChannel})
 
 	session := createMockSession(1, server)