markdown: ::audio / ::youtube / ::soundcloud directives — native player tag + click-to-render embed facades

Directive target lives in the label (micromark's extension has no
::name[label](url) destination form); titles with parens/dashes must be
quoted in attributes. Facades ship as static HTML — YouTube touches
i.ytimg.com for the still only, SoundCloud touches nothing — and
ArticleEnhance swaps in the nocookie/w.soundcloud player on click.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>