Block-level HTML tags in notebook markdown cells render incorrectly

Description

Block-level HTML elements like <details> and <summary> don't render correctly in notebook markdown cells. Content that should be hidden behind a collapsible disclosure is always visible.

This happens because marked splits block-level HTML into separate tokens when blank lines appear inside them. The split tokens render as siblings instead of as children of the outer element.

For example, this markdown:

<details>
<summary>Click to expand</summary>

Hidden content here

</details>

renders with "Hidden content here" always visible, outside the <details> element.

A secondary issue: double-clicking the <details> disclosure arrow enters edit mode instead of toggling the disclosure twice.

Expected behavior

• Block-level HTML elements render as a single grouped element, matching JupyterLab and Marimo
• <details> content is hidden by default and revealed on click
• Double-clicking the disclosure arrow toggles it without entering edit mode

Environment

Positron (main branch)