fix(mdx): collapse blank lines between JSX table elements on serialize

[jesseyowell]

Problem

When the MDX serializer outputs a JSX table (i.e. a table with flow content like lists, code blocks, or paragraph breaks in cells), blank lines are introduced between every pair of sibling table elements:

    </th>
                        <!-- blank line inserted here -->
    <th style={{ textAlign: "center" }}>

This happens between </th> ↔ <th>, </td> ↔ <td>, </thead> ↔ <tbody>, and </tr> ↔ <tr>. The MDX renderer treats these blank lines as new AST nodes, which breaks table structure on re-parse.

Root cause

mdast-util-mdx-jsx's containerFlow function unconditionally inserts \n\n between all children of flow JSX elements. This is correct for most block elements, but for table sub-elements (<thead>, <tbody>, <tr>, <th>, <td>) it produces whitespace that breaks the table when the MDX is later parsed.

Fix

Post-process the serialized MDX string in lib/mdx.ts to collapse \n\n → \n specifically between closing and opening table element tags:

return string.replace(
  /(<\/(?:th|td|tr|thead|tbody)>)\n\n(\s*<(?:th|td|tr|thead|tbody)[\s>])/g,
  '$1\n$2',
);

The regex only matches tag boundaries, so blank lines inside cell content (intentional paragraph breaks) are preserved.

Before

<Table align={["center","center"]}>
  <thead>
    <tr>
      <th style={{ textAlign: "center" }}>
        Header 1
      </th>

      <th style={{ textAlign: "center" }}>
        Header 2
      </th>
    </tr>
  </thead>

  <tbody>
    ...
  </tbody>
</Table>

After

<Table align={["center","center"]}>
  <thead>
    <tr>
      <th style={{ textAlign: "center" }}>
        Header 1
      </th>
      <th style={{ textAlign: "center" }}>
        Header 2
      </th>
    </tr>
  </thead>
  <tbody>
    ...
  </tbody>
</Table>

Tests

All 1993 tests pass. 11 inline snapshots updated across 4 test files to reflect the removed blank lines:

• __tests__/compilers/tables.test.js — 6 snapshots (core table serialization)
• __tests__/migration/tables.test.ts — 3 snapshots (RDMD → MDX migration)
• __tests__/compilers/compatability.test.tsx — 1 snapshot
• __tests__/migration/link-reference.test.ts — 1 snapshot

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