Add Persistable State

Add a persistable state

Two common bugs: a state that silently won't (de)serialize because its type was never registered, and a state that never gets saved because it wasn't added to the central SaveService. This skill prevents both. Read the FileHandler and SaveService sections in src/Game/Common/README.md and use SaveService.cs as the reference.

Steps:

1. Define the state as a plain POCO in <Module>/Domain/<Name>State.cs (public get/set properties, sane defaults, no logic, no Godot). If it lives in a CollectionRepository, give it an int Id (indexes/deserialization rely on it).

2. Put it in a repository (SingleRepository<T> or CollectionRepository<TKey,TValue>), created and seeded in GameLevel.

3. Register the type in GameLevel next to the others:

   _jsonConverterStateService.Register(typeof(<Name>State));
   

   Without this line the serializer skips it. This is the step people forget — do not skip it.

4. Add the repository to SaveService. Saving is centralized — do NOT write file I/O in the module service. Take the new repository in the SaveService constructor and add it to both methods, keeping the list order identical:

   // Save():  single state → GetOne(), collection → GetAll()
   var data = new List<object?> { _idRepository.GetOne(), ..., _<name>Repository.GetOne() };
   // Load():  single → Delete()+Update((T)data[i]!);  collection → DeleteAll()+loop Update(key,(T)value!)
   

   Bump the data is not { Count: N } guard in Load to the new slot count. Pass the repository into SaveService from GameLevel. Also reset it in NewGameService (Delete()+Update(new ...) for single, DeleteAll() for collections) so "new game" clears it too.

5. Custom field types (not handled by System.Text.Json) need a converter like Vector2JsonConverter registered in JsonConverterStateService.

6. Observable state? If the state is observable, remember SaveService.Load replaces the object — subscribers must re-subscribe after a load (see add-observer).

7. Test with a mocked IJsonStateFileHandlerService (see SaveServiceTests and add-tests), then run verify.