feat(core): compose Affine → Getter (Optional/AffineFold andThen Getter = AffineFold)

[kryptt]

Summary

Composing a partial optic (Optional / AffineFold, the Affine carrier) with a read-only Getter now works and yields an AffineFold (a partial read): getOption the focus, then read it through the getter.

val ageOpt: Optional[Doc, Doc, Int, Int] = Optional(...)
val toStr: DirectGetter[Int, String] = Getter[Int, String](_.toString)

val partialRead: AffineFold[Doc, String] = ageOpt.andThen(toStr)   // "optionally focus Int, then read String"
partialRead.getOption(doc)   // Some("20") on hit, None on miss

Why a downgrade to AffineFold

A read-write/partial outer has back-focus B = A, but a Getter's is Unit — so a Getter inner can never thread through as a writable optic. The read-only downgrade is the only sound result, and it's exactly what you want. It mirrors how a Getter chain composes Getter-to-Getter (a read-only inner collapses the result to read-only).

Implementation

• Optional (concrete, read-write) andThen Getter — a new fused member on the Optional class, alongside its existing andThen(Optional/Lens/Prism/Iso) members, returning AffineFold[S, C]. Deliberately a member, not an extension: an extension andThen on the Affine receiver perturbed the existing cross-carrier resolution (it broke Affine.andThen(MultiFocus)); a member with a distinct DirectGetter param type is resolved by argument type and leaves every other andThen untouched.
• AffineFold (read-only) andThen Getter — already composes via the morphed path (Morph[Affine, Direct] → AffineFold); now covered by an explicit test.

Testing

• OpticsBehaviorSpec: a new case asserting both outers (Optional and AffineFold) compose with a Getter to an AffineFold, hit and miss. 22/22 in that spec.
• Full sbt test aggregate green (the change touches andThen resolution, so the whole optic + law suite was exercised — the previously-passing Affine.andThen(MultiFocus) case still passes).
• Docs: a note in optics.md under Optional.

Scope

Independent core optic-algebra capability — does not depend on the typed-recursion-schemes work in #24. The Lens → Getter sibling (read-only downgrade for the Tuple2 carrier) is the obvious parallel and a natural follow-up; this PR does the Affine carrier as requested.

Post-Deploy Monitoring & Validation

No additional operational monitoring required — pure, additive library change (a new fused andThen overload), no runtime/external/deployment impact.

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