Render Unicode math in pandoc PDFs with tectonic engine

When pandoc has no pdflatex/xelatex/wkhtmltopdf, check for tectonic (a self-contained XeTeX engine): --pdf-engine=tectonic just works with no TeX install. Gotcha: its default Latin Modern fonts silently drop Unicode math glyphs (U+2260 not-equal, U+2194 arrow) with only a warning. Instead of configuring fontspec, sed-preprocess a temp copy to swap those glyphs for LaTeX math ($\neq$, $\leftrightarrow$) so the source markdown stays readable and the PDF renders clean.