docs(readme): Studio's PDF export needs LaTeX; typst only covers plain pandoc

The README said /studio-pdf and studio_export_pdf use typst (since v1.4.0)
and that "PDF export from Studio/pandoc works out of the box" (since
v1.9.1). Only the pandoc half is true: pi-studio uses $PANDOC_PDF_ENGINE,
falling back to xelatex, and the image has never set it, so Studio export
fails with "xelatex not found" (seen at v1.6.3 and v1.11.2).

New "PDF export from Studio needs LaTeX" section:
- do not set PANDOC_PDF_ENGINE=typst: pi-studio's LaTeX header is then
  printed as page-1 text while pandoc exits 0
- texlive-xetex alone is not enough (lmodern.sty, ulem.sty, pzdr font);
  texlive-xetex lmodern texlive-plain-generic texlive-fonts-recommended
  works, 37 packages / 491 MB, measured on v1.11.2 + pi-studio 0.10.1 by
  exporting and rendering the page

AGENTS.md and Dockerfile.base comments carry the same claim; not changed.

CHANGELOG.md

@@ -66,6 +66,33 @@ base** rather than reusing `base-6f3b20266de1`.
   read-only preview, and an on-host `curl` that tells "chain up" (`403`) from
   "Studio down or on another port" (empty reply). A remote session hit exactly
   that: Studio was started on `18765` while only `8765` is published.
+- **README said PDF export from Studio works out of the box. It does not.**
+  Since v1.4.0 (`67da05b`) the README's Studio section has said that
+  `/studio-pdf` and `studio_export_pdf` use `typst`. Since v1.9.1
+  (`6353d59`) the image-tags section has also said that "PDF export from
+  Studio/pandoc works out of the box". That holds for pandoc run directly
+  (`pandoc doc.md --pdf-engine=typst -o doc.pdf`), but not for Studio.
+  pi-studio runs pandoc with `$PANDOC_PDF_ENGINE`, falling back to `xelatex`,
+  and the image has never set that variable (`git log -S PANDOC_PDF_ENGINE`
+  finds no commit). So `/studio-pdf` and `studio_export_pdf` fail with
+  `xelatex not found`. That was seen at v1.6.3 and again at v1.11.2. The
+  README now says so in both places that made the claim, and adds a
+  "PDF export from Studio needs LaTeX" section with two warnings and a
+  working recipe:
+  - **Setting `PANDOC_PDF_ENGINE=typst` makes it worse.** The error message
+    suggests it, but pi-studio adds its own LaTeX header to the document
+    (unless the document sets `header-includes`). Under typst, pandoc exits
+    0 and prints that header as text above the first heading. Seen in the same PDF through two renderers, MuPDF and
+    Chromium.
+  - **`texlive-xetex` alone, which the README offered as the fallback, is
+    not enough.** The export then still lacks `lmodern.sty`, `ulem.sty`
+    and the `pzdr` font. Adding `lmodern texlive-plain-generic
+    texlive-fonts-recommended` makes it work: 37 packages and 491 MB
+    installed, after `apt-get update`. Measured on v1.11.2 with pi-studio
+    0.10.1. The exported page was looked at, not only its exit code.
+
+  The same claim also appears in `AGENTS.md` ("What we DON'T install") and
+  in comments in `Dockerfile.base`. Neither is changed here.
 
 ## v1.11.2 — 2026-10-14
 

README.md

@@ -175,10 +175,12 @@ Currently published:
 | `joakimp/pi-devbox:latest-studio` | `latest` + [pi-studio](https://github.com/omaclaren/pi-studio) (browser prompt editor, KaTeX/Mermaid preview, tmux-backed literate REPLs) | ~3.25 GB |
 | `joakimp/pi-devbox:vX.Y.Z-studio` | pinned-version studio equivalent | ~3.25 GB |
 
-Both variants ship **`typst`** as the pandoc PDF engine
+Both variants ship **`typst`**, a single ~30 MB static binary, as a PDF engine
-(`pandoc --pdf-engine=typst`), a single ~30 MB static binary, so PDF export from
+for pandoc, so `pandoc doc.md --pdf-engine=typst -o doc.pdf` works out of the
-Studio/pandoc works out of the box — no separate `-tex` variant needed.
+box (see [Generating a PDF with pandoc + typst](#generating-a-pdf-with-pandoc--typst)).
-`texlive-xetex` stays the higher-fidelity fallback (install on demand).
+**pi-studio's PDF export does not use typst.** `/studio-pdf` and
+`studio_export_pdf` need a LaTeX engine, which neither variant ships (see
+[PDF export from Studio needs LaTeX](#pdf-export-from-studio-needs-latex)).
 
 ## Using pi-studio (`-studio` variant)
 
@@ -345,10 +347,43 @@ Assuming the compose file publishes `127.0.0.1:8765:8765` (see method B):
 > an empty reply means the bridge has nothing behind it, i.e. Studio is down
 > or on another port.
 
-> PDF export (`/studio-pdf`, `studio_export_pdf`) uses **`typst`**, shipped in
+### PDF export from Studio needs LaTeX
-> the base image as the pandoc PDF engine (`pandoc --pdf-engine=typst`). For
+
-> LaTeX-exact output you can install `texlive-xetex` on demand as a heavier
+Studio's PDF export (`/studio-pdf` and the `studio_export_pdf` tool) fails out
-> fallback. HTML export, KaTeX, Mermaid, and all REPL features work regardless.
+of the box:
+
+```text
+pandoc PDF export failed with exit code 47: xelatex not found
+```
+
+pi-studio runs pandoc with the engine named in `PANDOC_PDF_ENGINE`, and uses
+`xelatex` when that is unset. The image ships no LaTeX engine and does not set
+`PANDOC_PDF_ENGINE`. HTML export, KaTeX, Mermaid and the REPLs are not
+affected.
+
+**Do not set `PANDOC_PDF_ENGINE=typst`**, even though the error message
+suggests setting it. pi-studio adds its own LaTeX header
+(`\usepackage{titlesec}`, `\titleformat…`) to the document, and typst does
+not understand LaTeX. pandoc exits 0, and the header is printed as text above
+your first heading. There is no error, only a wrong PDF.
+
+There are two ways out:
+
+- **Run pandoc yourself** (nothing to install):
+  `pandoc doc.md --pdf-engine=typst -o doc.pdf`. See the next section.
+- **Install the LaTeX packages Studio needs.** This adds 37 packages and
+  about 490 MB, and they are gone when the container is recreated:
+
+  ```bash
+  sudo apt-get update
+  sudo apt-get install -y --no-install-recommends \
+    texlive-xetex lmodern texlive-plain-generic texlive-fonts-recommended
+  ```
+
+  `texlive-xetex` on its own is not enough. Studio's export then still lacks
+  `lmodern.sty`, `ulem.sty` and the `pzdr` font, which come from the other
+  three packages. The image starts with an empty package index, so run
+  `apt-get update` first. (Measured on v1.11.2 with pi-studio 0.10.1.)
 
 ### Generating a PDF with pandoc + typst