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_core_test.go

@@ -572,7 +572,7 @@ func TestHandleMsgSysAuthTerminal(t *testing.T) {
 func TestHandleMsgSysLockGlobalSema_NoMatch(t *testing.T) {
 	server := createMockServer()
 	server.GlobalID = "test-server"
-	server.Channels = []*Server{}
+	server.Registry = NewLocalChannelRegistry([]*Server{})
 	session := createMockSession(1, server)
 
 	pkt := &mhfpacket.MsgSysLockGlobalSema{
@@ -602,7 +602,7 @@ func TestHandleMsgSysLockGlobalSema_WithChannel(t *testing.T) {
 		GlobalID: "other-server",
 	}
 	channel.stages.Store("stage_user123", NewStage("stage_user123"))
-	server.Channels = []*Server{channel}
+	server.Registry = NewLocalChannelRegistry([]*Server{channel})
 
 	session := createMockSession(1, server)
 
@@ -633,7 +633,7 @@ func TestHandleMsgSysLockGlobalSema_SameServer(t *testing.T) {
 		GlobalID: "test-server",
 	}
 	channel.stages.Store("stage_user456", NewStage("stage_user456"))
-	server.Channels = []*Server{channel}
+	server.Registry = NewLocalChannelRegistry([]*Server{channel})
 
 	session := createMockSession(1, server)