#752 feat: mark horizontal scrollers with `data-scroll-x`

Code blocks and math and Mermaid previews scroll horizontally on their own. They now carry a stable data-scroll-x attribute in the editor and the read-only MarkdownView, so a host page can select them without test IDs or hashed CSS module classes. For example, a host whose content only pans vertically can give [data-scroll-x] back touch-action: auto. The core toDOM leaves it out, so copied HTML stays clean.

Summary by CodeRabbit

• New Features
  • Code blocks, math renderings, and Mermaid diagrams are now marked for horizontal scrolling.