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/user_binary_store.go

@@ -0,0 +1,56 @@
+package channelserver
+
+import "sync"
+
+// userBinaryPartID is the composite key for a user binary part.
+type userBinaryPartID struct {
+	charID uint32
+	index  uint8
+}
+
+// UserBinaryStore is a thread-safe store for per-character binary data parts.
+type UserBinaryStore struct {
+	mu   sync.RWMutex
+	data map[userBinaryPartID][]byte
+}
+
+// NewUserBinaryStore creates an empty UserBinaryStore.
+func NewUserBinaryStore() *UserBinaryStore {
+	return &UserBinaryStore{data: make(map[userBinaryPartID][]byte)}
+}
+
+// Get returns the binary data for the given character and index.
+func (s *UserBinaryStore) Get(charID uint32, index uint8) ([]byte, bool) {
+	s.mu.RLock()
+	data, ok := s.data[userBinaryPartID{charID: charID, index: index}]
+	s.mu.RUnlock()
+	return data, ok
+}
+
+// GetCopy returns a copy of the binary data, safe for use after the lock is released.
+func (s *UserBinaryStore) GetCopy(charID uint32, index uint8) []byte {
+	s.mu.RLock()
+	src := s.data[userBinaryPartID{charID: charID, index: index}]
+	if len(src) == 0 {
+		s.mu.RUnlock()
+		return nil
+	}
+	dst := make([]byte, len(src))
+	copy(dst, src)
+	s.mu.RUnlock()
+	return dst
+}
+
+// Set stores binary data for the given character and index.
+func (s *UserBinaryStore) Set(charID uint32, index uint8, data []byte) {
+	s.mu.Lock()
+	s.data[userBinaryPartID{charID: charID, index: index}] = data
+	s.mu.Unlock()
+}
+
+// Delete removes binary data for the given character and index.
+func (s *UserBinaryStore) Delete(charID uint32, index uint8) {
+	s.mu.Lock()
+	delete(s.data, userBinaryPartID{charID: charID, index: index})
+	s.mu.Unlock()
+}