feat: add SavedVariables persistence for Lua addons

Addons can now persist data across sessions using the standard WoW
SavedVariables pattern:

1. Declare in .toc: ## SavedVariables: MyAddonDB
2. Use the global in Lua: MyAddonDB = MyAddonDB or {default = true}
3. Data is automatically saved on logout and restored on next login

Implementation:
- TocFile::getSavedVariables() parses comma-separated variable names
- LuaEngine::loadSavedVariables() executes saved .lua file to restore globals
- LuaEngine::saveSavedVariables() serializes Lua tables/values to valid Lua
- Serializer handles tables (nested), strings, numbers, booleans, nil
- Save triggered on PLAYER_LEAVING_WORLD and AddonManager::shutdown()
- Files stored as <AddonDir>/<AddonName>.lua.saved

Updated HelloWorld addon to track login count across sessions.

src/core/application.cpp

@@ -693,6 +693,7 @@ void Application::setState(AppState newState) {
             // If we reuse a previously spawned instance without forcing a respawn, appearance (notably hair) can desync.
             if (addonManager_ && addonsLoaded_) {
                 addonManager_->fireEvent("PLAYER_LEAVING_WORLD");
+                addonManager_->saveAllSavedVariables();
             }
             npcsSpawned = false;
             playerCharacterSpawned = false;