feat: add WoW event system for Lua addons

Implement the WoW-compatible event system that lets addons react to
gameplay events in real-time:

- RegisterEvent(eventName, handler) — register a Lua function for an event
- UnregisterEvent(eventName, handler) — remove a handler
- fireEvent() dispatches events to all registered handlers with args

Currently fired events:
- PLAYER_ENTERING_WORLD — after addons load and world entry completes
- PLAYER_LEAVING_WORLD — before logout/disconnect

Events are stored in a __WoweeEvents Lua table, dispatched via
LuaEngine::fireEvent() which is called from AddonManager::fireEvent().
Error handling logs Lua errors without crashing.

Updated HelloWorld addon to use RegisterEvent for world entry/exit.

include/addons/addon_manager.hpp

@@ -17,6 +17,7 @@ public:
     void scanAddons(const std::string& addonsPath);
     void loadAllAddons();
     bool runScript(const std::string& code);
+    void fireEvent(const std::string& event, const std::vector<std::string>& args = {});
     void shutdown();
 
     const std::vector<TocFile>& getAddons() const { return addons_; }