game: fix NPCs not spawning on reconnect to same map

On disconnect/reconnect to the same map, entityManager was not cleared
and creatureInstances_ still held old entries from the previous session.
When the server re-sent CREATE_OBJECT for the same GUIDs, the spawn
callback's early-return guard (creatureInstances_.count(guid)) silently
dropped every NPC re-spawn, leaving the world empty.

Fixes:
- disconnect() now calls entityManager.clear() to purge stale entities
- WorldEntryCallback gains a bool isInitialEntry parameter (true on first
  login or reconnect, false on in-world teleport/flight landing)
- Same-map optimization path skipped when isInitialEntry=true, so
  loadOnlineWorldTerrain runs its full cleanup and properly despawns old
  creature/player instances before the server refreshes them

include/game/game_handler.hpp

@@ -666,8 +666,8 @@ public:
     uint32_t getSkillCategory(uint32_t skillId) const;
 
     // World entry callback (online mode - triggered when entering world)
-    // Parameters: mapId, x, y, z (canonical WoW coordinates)
-    using WorldEntryCallback = std::function<void(uint32_t mapId, float x, float y, float z)>;
+    // Parameters: mapId, x, y, z (canonical WoW coords), isInitialEntry=true on first login or reconnect
+    using WorldEntryCallback = std::function<void(uint32_t mapId, float x, float y, float z, bool isInitialEntry)>;
     void setWorldEntryCallback(WorldEntryCallback cb) { worldEntryCallback_ = std::move(cb); }
 
     // Unstuck callback (resets player Z to floor height)