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/lua_engine.hpp

@@ -35,6 +35,10 @@ public:
     // Call OnUpdate scripts on all frames that have one.
     void dispatchOnUpdate(float elapsed);
 
+    // SavedVariables: load globals from file, save globals to file
+    bool loadSavedVariables(const std::string& path);
+    bool saveSavedVariables(const std::string& path, const std::vector<std::string>& varNames);
+
     lua_State* getState() { return L_; }
     bool isInitialized() const { return L_ != nullptr; }