fix: delegate all 113 stale GameHandler getters to domain handlers

PR #23 split GameHandler into 8 domain handlers but left 113 inline
getters reading stale duplicate member variables. Every feature that
relied on these getters was silently broken (showing empty/stale data):

InventoryHandler (32): bank, mail, auction house, guild bank, trainer,
  loot rolls, vendor, buyback, item text, master loot candidates
SocialHandler (43): guild roster, battlegrounds, LFG, duels, petitions,
  arena teams, instance lockouts, ready check, who results, played time
SpellHandler (10): talents, craft queue, GCD, pet unlearn, queued spell
QuestHandler (13): quest log, gossip POIs, quest offer/request windows,
  tracked quests, shared quests, NPC quest statuses
MovementHandler (15): all 8 server speeds, taxi state, taxi nodes/data

All converted from inline `{ return member_; }` to out-of-line
delegations: `return handler_ ? handler_->getter() : fallback;`

include/game/social_handler.hpp

@@ -205,6 +205,11 @@ public:
     // Instance lockouts
     const std::vector<InstanceLockout>& getInstanceLockouts() const { return instanceLockouts_; }
 
+    // Instance difficulty
+    uint32_t getInstanceDifficulty() const { return instanceDifficulty_; }
+    bool isInstanceHeroic() const { return instanceIsHeroic_; }
+    bool isInInstance() const { return inInstance_; }
+
     // Minimap ping
     void sendMinimapPing(float wowX, float wowY);