MarkdownWidget::render() can be used directly with a standalone RenderContext, but sub-element styles were only resolved when the widget was attached to a TUI tree. In that standalone case, AbstractWidget::resolveElement() returned an empty style, so Markdown headings, bold text, italic text, links, and other styled inline elements silently rendered without their default styling.

This changes the fallback at the widget level: when a widget is not attached to a context, sub-element styles are resolved from the default TUI stylesheet. That keeps standalone rendering consistent with attached rendering for core widgets.

For example:

$markdown = new MarkdownWidget("# Hello\n\nsome **bold** and *italic* text");
$lines = $markdown->render(new RenderContext(60, 20));

Before, the heading kept the raw Markdown marker and inline formatting lost its ANSI styling:

# Hello

some bold and italic text

After, the heading content is rendered without the Markdown syntax marker and default ANSI styles are applied to the heading, bold, and italic spans.

A regression test covers rendering an unattached MarkdownWidget directly with RenderContext.