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.

src/addons/addon_manager.cpp

@@ -85,6 +85,10 @@ bool AddonManager::runScript(const std::string& code) {
     return luaEngine_.executeString(code);
 }
 
+void AddonManager::fireEvent(const std::string& event, const std::vector<std::string>& args) {
+    luaEngine_.fireEvent(event, args);
+}
+
 void AddonManager::shutdown() {
     addons_.clear();
     luaEngine_.shutdown();