fix(base): default pandoc typst template font so PDF export works without -V mainfont

pandoc's bundled typst template defaults the document font to an empty
tuple (font: ()), so a naked `pandoc --pdf-engine=typst` fails with
"font fallback list must not be empty". Patch the template default to
Libertinus Serif (typst's own bundled default) at build time so PDF
export works out of the box. Document usage in README and note the fix
in CHANGELOG (Unreleased). Base-affecting.

README.md

@@ -292,6 +292,26 @@ Assuming the compose file publishes `127.0.0.1:8765:8765` (see method B):
 > LaTeX-exact output you can install `texlive-xetex` on demand as a heavier
 > fallback. HTML export, KaTeX, Mermaid, and all REPL features work regardless.
 
+### Generating a PDF with pandoc + typst
+
+The base ships `pandoc` (front-end) and `typst` (PDF engine), so Markdown → PDF
+works out of the box:
+
+```bash
+pandoc doc.md --pdf-engine=typst -o doc.pdf
+```
+
+The base patches pandoc's bundled typst template so it defaults to the
+**Libertinus Serif** font. Without that patch a naked `--pdf-engine=typst`
+fails with `error: font fallback list must not be empty`, because the upstream
+template leaves the font unset. To pick a different face, pass one of the fonts
+typst can see (`typst fonts` lists them — DejaVu Serif/Sans/Mono, Libertinus
+Serif, New Computer Modern):
+
+```bash
+pandoc doc.md --pdf-engine=typst -V mainfont="New Computer Modern" -o doc.pdf
+```
+
 ### Graphviz diagrams in Studio: `dot-watch`
 
 pi-studio renders **Mermaid** natively but has **no Graphviz/DOT renderer**.