fix(channelserver): eliminate data races in shutdown and session lifecycle

The channel server had several concurrency issues found by the race
detector during isolation testing:

- acceptClients could send on a closed acceptConns channel during
  shutdown, causing a panic. Replace close(acceptConns) with a done
  channel and select-based shutdown signaling in both acceptClients
  and manageSessions.
- invalidateSessions read isShuttingDown and iterated sessions without
  holding the lock. Rewrite with ticker + done channel select and
  snapshot sessions under lock before processing timeouts.
- sendLoop/recvLoop accessed global _config.ErupeConfig.LoopDelay
  which races with tests modifying the global. Use the per-server
  erupeConfig instead.
- logoutPlayer panicked on DB errors and crashed on nil DB (no-db
  test scenarios). Guard with nil check and log errors instead.
- Shutdown was not idempotent, double-calling caused double-close
  panic on done channel.

Add 5 channel isolation tests verifying independent shutdown,
listener failure, session panic recovery, cross-channel registry
after shutdown, and stage isolation.

server/channelserver/sys_session.go

@@ -4,7 +4,6 @@ import (
 	"encoding/binary"
 	"encoding/hex"
 	"erupe-ce/common/mhfcourse"
-	_config "erupe-ce/config"
 	"fmt"
 	"io"
 	"net"
@@ -172,7 +171,7 @@ func (s *Session) sendLoop() {
 				s.logger.Warn("Failed to send packet", zap.Error(err))
 			}
 		}
-		time.Sleep(time.Duration(_config.ErupeConfig.LoopDelay) * time.Millisecond)
+		time.Sleep(time.Duration(s.server.erupeConfig.LoopDelay) * time.Millisecond)
 	}
 }
 
@@ -215,7 +214,7 @@ func (s *Session) recvLoop() {
 			return
 		}
 		s.handlePacketGroup(pkt)
-		time.Sleep(time.Duration(_config.ErupeConfig.LoopDelay) * time.Millisecond)
+		time.Sleep(time.Duration(s.server.erupeConfig.LoopDelay) * time.Millisecond)
 	}
 }