feat: Deprecate internal state properties of Shanten

[Apricot-S]

This PR deprecates all public attributes of Shanten other than AGARI_STATE.

• These attributes were previously exposed as public fields but actually represent internal state used during backtracking.

• Accessing them now triggers a DeprecationWarning with the message:

  "`<name>` is deprecated. This attribute reflects internal state and should not be used."

- No setter is provided; attempts to assign to these attributes will raise AttributeError.

Benefits of this change

• Improves API clarity
• Prevents misuse of unstable internal values

[Nihisil]

Thank you!