Hide post-login world hitch behind loading screen

- keep character-selection state until world entry load/finalize completes
- move expensive post-load setup (test transport + creature callback prep) before loading screen shutdown
- add bounded world warmup pass under loading screen to drain initial network/spawn backlog
- start intro camera pan after warmup so rotation begins when gameplay becomes visible
- guard test transport setup so it runs once per session
- add per-update world socket parse budget to prevent single-frame packet-drain stalls

This reduces visible 3-4s stutter after login by shifting startup work behind the loading screen and time-slicing packet processing.

include/core/application.hpp

@@ -218,6 +218,7 @@ private:
     std::unordered_map<uint64_t, PendingTransportMove> pendingTransportMoves_; // guid -> latest pre-registration move
     uint32_t nextGameObjectModelId_ = 20000;
     uint32_t nextGameObjectWmoModelId_ = 40000;
+    bool testTransportSetup_ = false;
     bool gameObjectLookupsBuilt_ = false;
 
     // Mount model tracking