mathml differences between lua-unicode-math and unicode-math

[mbertucci47]

There are several significant differences in the mathml produced by unicode-math and lua-unicode-math. I opened #1265 for a convenient place to view the diffs in the test xml files when unicode-math is replaced with lua-unicode-math. I mentioned the following in that PR but I will also list them here, and I'm sure there are several more differences.

• The prime character ′ in mathml goes from <mo lspace="0" rspace="0">′</mo> to <mi mathvariant="normal">′</mi>. Same for !.
• Sometimes several MCs get collapsed into one, sometimes the other way around.
• Since all the mathml is put on one line, some of the diffs are very difficult to parse. I wonder how difficult it would be to provide better formatting of the mathml. See, for one example, amsmath-01.
• I did not include any changes that are due to changes in luatex and luamml (see comments in Update some tests for texlive 2026 #1254).
• In at least one case an entire AssociatedFile disappears; see numerica-01. In another an entire Formula disappears; see longdivision-01.
• In some cases characters change, e.g. extarrows-01.
• In letterswitharrows-01, <mover> <mi mathvariant="normal">x</mi> <mo stretchy="false">→</mo> </mover> becomes <mi> <mglyph/> </mi> which definitely seems wrong.