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.

CHANGELOG.md

@@ -11,6 +11,23 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 ---
 
+## Unreleased
+
+### Fixed
+
+- **`pandoc --pdf-engine=typst` now works without `-V mainfont`.** pandoc's
+  bundled typst template (`/usr/share/pandoc/data/templates/template.typst`)
+  defaults the document font to an empty tuple (`font: ()`), so a naked
+  `pandoc --pdf-engine=typst` (and `studio_export_pdf` in some cases) failed
+  with `error: font fallback list must not be empty` unless the caller passed
+  `-V mainfont="..."`. The base now patches that template default to
+  `Libertinus Serif` (typst's own bundled default font) at build time, so PDF
+  export works out of the box. Base-affecting (`Dockerfile.base` RUN), rebuilds
+  `base-<hash>`. README gains a "Generating a PDF with pandoc + typst" section
+  with the working command and how to override the font via `-V mainfont`.
+
+---
+
 ## v1.4.0 — 2026-07-11
 
 Minor release. Headline: **PDF export works out of the box** — the base now