add VisibleForReflection

So, I've been thinking about how one could warn a user that a constructor or method really only exists for reflective access. This is mostly an issue when dealing within the same package these days.

JPA for example requires a public/protected constructor. I personally am always going to use a protected one, but that still shouldn't be called outside of subclasses, or maybe tests.

So my thought is an annotation like `VisibleForReflection` would be nice.

The issue names no source files, tests, or implementation entry point. First clarify the intended annotation behavior and warning mechanism, including how reflective access and legitimate subclasses or tests should be distinguished; done should include an agreed API and corresponding validation.

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