resource-saver-loader recipe out of date

src/recipes/resource-saver-loader.md uses `on_level_init` and `on_level_deinit` but according to src/migrate/v0.5.md it should be `on_stage_init` and `on_stage_deinit`. `MyAssetSingleton::class_name` should be `MyAssetSingleton::class_id`. src/recipes/engine-singleton.md uses the correct names

I have not checked other recipes

Start with src/recipes/resource-saver-loader.md and compare its names against src/migrate/v0.5.md; use src/recipes/engine-singleton.md as a known-correct example. Check the other recipe files for the same outdated names, and finish when the affected recipes consistently use the current API names.

Written by the indexing model from the issue text.