fix(math): preserve MathML when flattening arXiv equation tables

[RovshanMuradov]

fix(math): preserve MathML when flattening arXiv equation tables

Why

Flattening arXiv equation tables currently replaces existing MathML with plain LaTeX text. HTML consumers can no longer render the original MathML and must reconstruct it from LaTeX, which may contain unsupported commands.

Change

Clone the existing <math> element instead of creating an empty element and assigning textContent = latex. Keep the existing display and data-latex assignments and subsequent cleanup.

This is a small change in src/standardize.ts, with no new dependencies or public API changes. It does not address the separate HTML wrapper cleanup problem reported in #377.

Example

An equation inside an arXiv-style table:

<table class="ltx_equation"><tr><td>
  <math display="block" alttext="x"><semantics>
    <mi>x</mi>
    <annotation encoding="application/x-tex">x</annotation>
  </semantics></math>
</td></tr></table>

Place this inside an article with surrounding text and extract it with the Node API using LinkeDOM and default standardization (markdown: false). Before this change, the resulting <math data-latex="x"> contains only plain x. Afterward, it retains <semantics><mi>x</mi>…</semantics> while the table wrapper is removed.

Verification

• All 454 existing default checks pass. The JSDOM run has the same 124 failing cases before and after the change.
• On ten arXiv articles, the isolated patch preserves structured MathML in 1,889 additional formulas without changing our downstream BlockNote JSON or Markdown.
• Defuddle's own Markdown is identical on nine articles. On the tenth, only ten whitespace-only lines between display equations disappear.
• Manual LinkeDOM examples cover nested equation groups, MathML-only and LaTeX-only inputs, and unsafe markup. Formula order is preserved without duplicates; the supplied scripts, event handlers, and javascript: URLs are removed.

These checks establish content preservation, not visual correctness of every formula. The existing no-LaTeX guard is unchanged.