[Don-Yin]

The Typst writer ignores NoLineFrac, so {a \atop b} and \genfrac{}{}{0pt}{}{a}{b} both become a / b and render with a fraction bar.

I render lineless fractions as a two-row matrix without delimiters: mat(delim: #none, a; b). Reusing the matrix cell writer also handles empty numerators and denominators. The existing binom special case and ordinary fractions are unchanged.

I added a Typst writer regression covering \atop, zero-width \genfrac, compound operands, brackets, and empty operands, and corrected two existing expectations that included unwanted bars.

Validation:

• GHC 9.10.3: the regression fails before the change; cabal test -fexecutable -f-server --test-show-details=direct passes all 3163 tests afterward.
• Typst 0.14.2: I compiled both examples to SVG and PNG before and after the change. Each had one fraction-bar stroke before and none afterward. I also compiled the compound, bracketed, empty-operand, binomial, and ordinary-fraction cases and checked their rendering; the ordinary fraction retains its bar.