refactor: extract updateTaxiAndMountState() from update()

Move 131 lines of taxi flight detection, mount reconciliation, taxi
activation timeout, and flight recovery into a dedicated method.
update() is now ~277 lines (61% reduction from original 704).

include/game/game_handler.hpp

@@ -2317,6 +2317,7 @@ public:
     void updateNetworking(float deltaTime);
     void updateTimers(float deltaTime);
     void updateEntityInterpolation(float deltaTime);
+    void updateTaxiAndMountState(float deltaTime);
 
     /**
      * Reset DBC-backed caches so they reload from new expansion data.