Does the default impl of `ProjectField::is_projectable` permit unsound union field projection?

> On second re-reading, I think this is actually unsound as written. Just because a union is valid doesn't mean that any particular *field* is valid. E.g. you could initialize a union with a field value `2u8` – that doesn't mean that another field of type `bool` is valid.

_Originally posted by @joshlf in https://github.com/google/zerocopy/pull/2940#discussion_r2733481797_