refactor(channelserver): extract UserBinaryStore and MinidataStore

The userBinary and minidata maps with their locks were spread across
Server as raw fields with manual lock management. Cross-channel session
searches also required acquiring nested locks (server lock + binary
lock). Encapsulating in dedicated types eliminates the nested locking
and reduces Server's field count by 4.

server/channelserver/minidata_store.go

@@ -0,0 +1,29 @@
+package channelserver
+
+import "sync"
+
+// MinidataStore is a thread-safe store for per-character enhanced minidata.
+type MinidataStore struct {
+	mu   sync.RWMutex
+	data map[uint32][]byte
+}
+
+// NewMinidataStore creates an empty MinidataStore.
+func NewMinidataStore() *MinidataStore {
+	return &MinidataStore{data: make(map[uint32][]byte)}
+}
+
+// Get returns the minidata for the given character ID.
+func (s *MinidataStore) Get(charID uint32) ([]byte, bool) {
+	s.mu.RLock()
+	data, ok := s.data[charID]
+	s.mu.RUnlock()
+	return data, ok
+}
+
+// Set stores minidata for the given character ID.
+func (s *MinidataStore) Set(charID uint32, data []byte) {
+	s.mu.Lock()
+	s.data[charID] = data
+	s.mu.Unlock()
+}