Fix online mode creature spawning and packet parsing

- Fix encryption desync by tracking decrypted header bytes in world socket
- Fix UPDATE_OBJECT movement block parsing to handle 3.3.5a update flags
- Fix UNIT_FIELD_DISPLAYID index (67, not 71)
- Add creature spawn/despawn callbacks with DBC-based model loading
- Add SMSG_COMPRESSED_UPDATE_OBJECT opcode support

include/game/entity.hpp

@@ -147,6 +147,10 @@ public:
     uint32_t getEntry() const { return entry; }
     void setEntry(uint32_t e) { entry = e; }
 
+    // Display ID (model display)
+    uint32_t getDisplayId() const { return displayId; }
+    void setDisplayId(uint32_t id) { displayId = id; }
+
 protected:
     std::string name;
     uint32_t health = 0;
@@ -156,6 +160,7 @@ protected:
     uint8_t powerType = 0;   // 0=mana, 1=rage, 2=focus, 3=energy
     uint32_t level = 1;
     uint32_t entry = 0;
+    uint32_t displayId = 0;
 };
 
 /**