refactor: extract updateEntityInterpolation() from update()

Move entity movement interpolation loop (distance-culled per-entity
update) into its own method. update() is now ~406 lines (down from
original 704, a 42% reduction across 3 extractions).

include/game/game_handler.hpp

@@ -2316,6 +2316,7 @@ public:
     void update(float deltaTime);
     void updateNetworking(float deltaTime);
     void updateTimers(float deltaTime);
+    void updateEntityInterpolation(float deltaTime);
 
     /**
      * Reset DBC-backed caches so they reload from new expansion data.