ReadCompose

Read-side composition across the read/write seam — the join that AssociativeFunctor (which needs B-threading on a shared carrier) structurally cannot express. Composes the READ halves of two optics, ignoring both write sides, and lands at the read-only join of their strengths:

 total   ∘ total   = Getter        (Accessor ∘ Accessor)
 total   ∘ partial = AffineFold    (any mix involving a PartialAccessor)
 partial ∘ total   = AffineFold
 partial ∘ partial = AffineFold
 Fold[F] ∘ total   = Fold[F]       (F has a cats Functor: the fold stays in its container)
 Fold[F] ∘ partial = Fold[F]       (F has a cats FunctorFilter: an inner miss is F's empty)
 many on either side = Fold        (ForgetfulFold fallback, List-backed)

The two Fold[F] cells keep the outer fold's container through single-focus inners — a fold into Option, Either[E, *] or an error-carrying result type stays one, with the inner read applied inside it, instead of being flattened to a List of foci.

Drives the read-collapse andThen members: the Optic trait's any-outer ∘ read-only-inner overload and the read-only-outer ∘ any-inner members on optics.Getter / optics.PickFold / optics.ForgetFold. Instances live in this companion, so they are in implicit scope for every carrier pair with no import.