chore: checkpoint stable battle and eidolon progression

internal/gameserver/session.go

@@ -29,8 +29,9 @@ type Session struct {
 	ConvID uint32
 	Addr   string
 
-	jsonMu sync.RWMutex
-	json   *common.FreesrData
+	jsonMu  sync.RWMutex
+	stateMu sync.Mutex // serializes packet handlers and hot reload mutations
+	json    *common.FreesrData
 
 	kcpMu sync.Mutex
 	kcp   *kcplib.KCP
@@ -45,7 +46,8 @@ type Session struct {
 	// attack actually hit a monster (the camera-detach bug fix).
 	MonsterEntities map[uint32]uint32
 	// Set by the challenge scene loader, cleared when loading an overworld scene.
-	ChallengeStageID uint32
+	ChallengeStageID  uint32
+	pendingExRankSync map[uint32]bool
 
 	closed    atomic.Bool
 	closeOnce sync.Once