Carousel buttons have the wrong or missing screen-reader label

• Where: carousel_previous.rb renders span.sr-only "Next slide";
  carousel_next.rb renders no sr-only span at all. Snapshot
  carousel/vertical line 17.
• Effect: a screen reader announces the previous button as "Next slide"
  and the next button as unlabeled.
• Fix: "Previous slide" on the previous button, add "Next slide" to the
  next button; re-record (on the 2.0 line) carousel/*.

Surfaced by the golden HTML suite (PR #536), which pins what 1.6 renders today — this defect included. Per maintainer decision (2026-09-19) main stays as is: this is addressed on the 2.0 line, and the golden snapshot that pins it is re-recorded there with the diff reviewed.

Source: design/v2/follow-up-issues.md.

Beginne mit carousel_previous.rb und carousel_next.rb, untersuche dann den Snapshot carousel/vertical in Zeile 17 sowie die anderen goldenen carousel/-Snapshots im 2.0-Zweig. Führe die goldene HTML-Suite aus und zeichne carousel/ neu auf; abgeschlossen ist die Aufgabe, wenn die Previous- und Next-Schaltflächen die korrekten Screenreader-Beschriftungen haben und der überprüfte Snapshot-Diff diese widerspiegelt.