EnumKp - A keypath for enum variants that supports both extraction and embedding
Leverages the existing Kp architecture where optionals are built-in via Option
AKp (AnyKeyPath) - Hides both Root and Value types
Most flexible keypath type for heterogeneous collections
Uses dynamic dispatch and type checking at runtime
Used so that then_async can infer V2 from AsyncKp::Value without ambiguity
(e.g. &i32 has both Borrow<i32> and Borrow<&i32>; this picks the referent).
Implemented only for reference types so there is no overlap with the blanket impl.