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/core/application.cpp

@@ -660,6 +660,9 @@ void Application::setState(AppState newState) {
             }
             // Ensure no stale in-world player model leaks into the next login attempt.
             // If we reuse a previously spawned instance without forcing a respawn, appearance (notably hair) can desync.
+            if (addonManager_ && addonsLoaded_) {
+                addonManager_->fireEvent("PLAYER_LEAVING_WORLD");
+            }
             npcsSpawned = false;
             playerCharacterSpawned = false;
             addonsLoaded_ = false;
@@ -5049,6 +5052,7 @@ void Application::loadOnlineWorldTerrain(uint32_t mapId, float x, float y, float
     if (addonManager_ && !addonsLoaded_) {
         addonManager_->loadAllAddons();
         addonsLoaded_ = true;
+        addonManager_->fireEvent("PLAYER_ENTERING_WORLD");
     }
 }