refactor(channelserver): replace global stagesLock with sync.Map-backed StageMap

The global stagesLock sync.RWMutex protected map[string]*Stage, causing
all stage operations to contend on a single lock even for unrelated
stages. Any stage creation or deletion blocked all reads server-wide.

Replace with a typed StageMap wrapper around sync.Map which provides
lock-free reads and allows concurrent writes to disjoint keys. Per-stage
sync.RWMutex remains unchanged for protecting individual stage state.

StageMap exposes Get, GetOrCreate, StoreIfAbsent, Store, Delete, and
Range methods. Updated ~50 call sites across 6 production files and
9 test files.

server/channelserver/handlers_clients.go

@@ -10,15 +10,12 @@ import (
 func handleMsgSysEnumerateClient(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgSysEnumerateClient)
 
-	s.server.stagesLock.RLock()
-	stage, ok := s.server.stages[pkt.StageID]
+	stage, ok := s.server.stages.Get(pkt.StageID)
 	if !ok {
-		s.server.stagesLock.RUnlock()
 		s.logger.Warn("Can't enumerate clients for stage that doesn't exist!", zap.String("stageID", pkt.StageID))
 		doAckSimpleFail(s, pkt.AckHandle, make([]byte, 4))
 		return
 	}
-	s.server.stagesLock.RUnlock()
 
 	// Read-lock the stage and make the response with all of the charID's in the stage.
 	resp := byteframe.NewByteFrame()