Second Attempt: Add <veil>, an inline reveal/hide element for guided …

[geoff-cox]

Hopefully, this is a clean merge. I tested it before the original PR, but didn't retest it. Perhaps you could take it for a spin, or I could later.

Summary

This PR introduces a new PreTeXt element, , to hide inline and block content until the "reveal" button is clicked. This version focuses more on the HTML side of the element and only offers minimal support for print.

HTML

• Renders a compact “reveal” button.
• On click/Enter/Space, hidden content is revealed inline with a thin accent border; clicking inside the revealed area hides it again.

LaTeX/Print

• Inline veils print as their content underlined via \uline{…}.
• Multiline/block veils are printed plainly (no underline).
• Veils inside math are not supported yet; content is printed as-is with a warning.
• Hiding content in print is not supported yet. Further dev discussion is necessary.

Testing

• Added a section in the sample-article.ptx containing a comprehensive list of test samples.

No schema or publisher-variable changes are included in this PR; those can follow in a subsequent change.