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.

include/addons/toc_parser.hpp

@@ -17,6 +17,7 @@ struct TocFile {
     std::string getTitle() const;
     std::string getInterface() const;
     bool isLoadOnDemand() const;
+    std::vector<std::string> getSavedVariables() const;
 };
 
 std::optional<TocFile> parseTocFile(const std::string& tocPath);