Select preview text by dragging, and arrow into nested code

- A press on a preview no longer lets the browser drop the caret into the
  hidden source. The selection follows the pointer inside the renderer's
  shadow roots, so code can be dragged over and copied without opening it.
  Copy from a preview is left to the browser. While the press lasts, the
  caret-follows-selection opening is suspended; the click decides.
- Arrow keys find a folded block across callouts and list items, not only
  top-level siblings, measuring from the DOM selection so a key pressed
  right after a click starts from the click.
- Escape that returns to a preview is not also taken by a comment preview.
- The focus ring sits on the block's border, clear of the corner controls.
- The syntax-contrast test checks the ring with expectFocusIndicator again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>