fix: use expansion context for spline parsing; preload DBC caches at world entry

Spline parsing: remove Classic format fallback from the WotLK parser. The
PacketParsers hierarchy already dispatches to expansion-specific parsers
(Classic/TBC/WotLK/Turtle), so the WotLK parseMovementBlock should only
attempt WotLK spline format. The Classic fallback could false-positive when
durationMod bytes resembled a valid point count, corrupting downstream parsing.

Preload DBC caches: call loadSpellNameCache() and 5 other lazy DBC caches
during handleLoginVerifyWorld() on initial world entry. This moves the ~170ms
Spell.csv load from the first SMSG_SPELL_GO handler to the loading screen,
eliminating the mid-gameplay stall.

WMO portal culling: move per-instance portalVisibleGroups vector and
portalVisibleGroupSet to reusable member variables, eliminating heap
allocations per WMO instance per frame.

include/game/game_handler.hpp

@@ -3406,6 +3406,7 @@ private:
     std::vector<TrainerTab> trainerTabs_;
     void handleTrainerList(network::Packet& packet);
     void loadSpellNameCache() const;
+    void preloadDBCCaches() const;
     void categorizeTrainerSpells();
 
     // Callbacks