Setext headings are parsed as paragraphs #33

Reproducer

(new Tempest\Markdown\Markdown(null))->parse("Price 2026\n-------------")->html;

Tempest 1.2.3 renders <p>Price 2026\n-------------</p>. A setext underline is a level-two heading in CommonMark, so the expected rendering is a heading. The same applies to = underlines for level one and to multiline heading content.

This occurs in existing Markdown documents and prevents a compatible renderer from using Tempest without a special case.