fix: ClassesOnlyAttribute is Runtime Attribute

[c3-hoge-fuga-piyo]

Proposed changes

Currently, ClassesOnlyAttribute resides in an editor-only assembly, which requires using #if UNITY_EDITOR guards when referenced from runtime code:

#if UNITY_EDITOR
[ClassesOnly]
#endif
[SerializeField] SerializableInterface<X> x;

This change moves ClassesOnlyAttribute into a runtime assembly, eliminating the dependency on editor assemblies and removing the need for conditional compilation in runtime code.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Refactoring (no functional changes, no api changes)
• [ ] Code style update (formatting, renaming)
• [ ] Build related changes (workflow changes)
• [ ] Documentation Update (readme, changelog)
• [ ] Other, please describe:

What is the current behavior?

Using ClassesOnlyAttribute requires referencing TNRD.SerializableInterface.Editor.asmdef, and for runtime code this necessitates an #if UNITY_EDITOR guard.

What is the new behavior?

Using ClassesOnlyAttribute will only require referencing TNRD.SerializableInterface.asmdef.

Checklist

• [x] The code compiles
• [x] I have tested that this doesn't break existing code (unless it is an explicit breaking change)
• [x] I have tested that the (new) functionality/fix works as intended