bd-attribute

Add a new declarative attribute to the better-data library (e.g. #[ArrayOf], #[Default], domain hint). Attributes live in src/Attribute/ as final readonly classes with constructor-promoted public properties — pure data carriers, never business logic. The failure mode that catches every contributor is "partial wiring" — declaring the attribute and reading it in ONE engine (e.g. only PostSink) while leaving Presenter, RestSchemaBuilder, and AttributeDrivenHydrator untouched. Stress scenarios have caught this pattern repeatedly. Every relevant engine must know about the new attribute, otherwise it silently degrades on the unwired path. Use when adding any new #[Foo] attribute that DTO authors will sprinkle on parameters / properties. Triggers on creating a class in src/Attribute/, applying #[Attribute(...)], references to AttributeDrivenHydrator / SinkProjection::prepareValue / RestSchemaBuilder / Presenter::sensitiveFieldNames in the diff.