Escaped LaTeX math retains & as an ordinary character, so matrix columns do not align. Restore its alignment catcode alongside the existing dollar, caret and underscore settings when mathescape, texcomments or escapeinside is enabled. Ordinary code ampersands keep their existing escaping.

Related to #1988.

Validation:

• Added regression cases for all three escape options and ordinary code; the three escape cases fail before the fix.
• tox -e py -- -W error: 5,334 passed, 16 skipped. tox -e check: passed.
• Independently reviewed and tested. Actual generated mathescape output with amsmath compiled using the desktop editor's XeTeX/Tectonic compiler: a numeric ampersand-catcode assertion fails on the baseline and passes with this change. The existing command-line LaTeX test remains skipped because the latex executable is unavailable.

Prepared with AI assistance and independently reviewed.

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