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c5b1960f - [ty] Share attribute write compatibility checks (#28555)

[ty] Share attribute write compatibility checks (#28555) ## Summary We move attribute-write compatibility checking from `protocol_class.rs` to `attribute_write.rs`, alongside the lookup and descriptor-setter helpers it uses. This makes the same write checks available to nominal attribute override validation. Protocol checking continues to use the active type relation and constraints. Descriptor setters, `Self` binding, class and instance fallbacks, and terminal `__setattr__` behavior are unchanged. Part of https://github.com/astral-sh/ty/issues/2158.