Introduce a quiz introduction block

This plugin drops you straight into the first question. There's no room for a title, a bit of framing, or a cover treatment ahead of showing the quiz itself.

We can work around this by providing surrounding content, but to give a quiz a proper opening that _avoids showing the first question_ until the user interacts, we need a block that owns that first slide.

A `gfo/quiz-intro` block could be a child of `hmquiz/quiz` that holds the quiz title, a short description, a start button, and a cover image. On the front end it would show first and keep the questions hidden until someone clicks start, then hand off to the quiz.

We could additionally make it the home for the cover image that each question falls back to when it has no image of its own, so the whole quiz has a sensible default in one place.

Keeping the intro as a dedicated block means it's real editable content the author fills in and styles like anything else (as opposed to hardcoding the title and framing into the quiz wrapper).

Start by reading the existing hmquiz/quiz block and how its first question is rendered. Define the gfo/quiz-intro child block around the requested title, description, start button, and cover image, then verify that it appears first, keeps questions hidden until interaction, and provides the quiz-wide image fallback.

Written by the indexing model from the issue text.