feat(base): ship typst as lightweight pandoc PDF engine

pandoc has been in the base since v1.0.0 but only as a front-end; PDF
export (studio_export_pdf / pandoc -o out.pdf) failed with 'xelatex not
found' because no back-end engine was installed. Ship typst (~30 MB
static Rust binary, no LaTeX) as the default engine via
`pandoc --pdf-engine=typst`, chosen over a ~600 MB TeX Live install.
texlive-xetex remains the higher-fidelity install-on-demand fallback.

- Dockerfile.base: install typst (latest GitHub-release idiom, pin via
  TYPST_VERSION); add xz-utils (typst ships .tar.xz); bump
  BASE_REBUILD_DATE. Lands in base-<hash>.
- smoke-test.sh: verify typst + a real pandoc --pdf-engine=typst render.
- README/AGENTS/CHANGELOG: typst now shipped (supersedes the planned
  :latest-studio-tex variant).

No tag pushed — CI build intentionally deferred.

README.md

@@ -149,12 +149,10 @@ Currently published:
 
 Planned for an upcoming minor release:
 
-- **PDF export from Studio/pandoc** (a future minor release). Engine still to be decided.
-  A lightweight engine is preferred over a full TeX install:
-  - `typst` — single static binary (~30 MB), `pandoc --pdf-engine=typst`; light
-    enough it could ship in the base image (no separate `-tex` variant needed).
-  - `joakimp/pi-devbox:latest-studio-tex` — `-studio` plus `texlive-xetex`
-    (~600 MB on top of `-studio`); highest fidelity, kept as the fallback.
+- *(shipped in Unreleased/base)* **PDF export from Studio/pandoc** now works:
+  the base image ships **`typst`** as the PDF engine (`pandoc --pdf-engine=typst`),
+  a single ~30 MB static binary — no separate `-tex` variant needed.
+  `texlive-xetex` stays the higher-fidelity fallback (install on demand).
 
 ## Using pi-studio (`-studio` variant)
 
@@ -289,11 +287,10 @@ Assuming the compose file publishes `127.0.0.1:8765:8765` (see method B):
 > until step 2 runs. If the browser can't connect, verify Studio is up
 > (`/studio --status`) and the bridge is running (`ps aux | grep socat`).
 
-> PDF export (`/studio-pdf`, `studio_export_pdf`) needs a PDF engine, which is
-> **not** in `-studio` (pandoc is present but has no engine, so export fails
-> with `xelatex not found`). Planned for a future release — likely a lightweight
-> engine such as `typst`, with `texlive-xetex` as the heavier fallback. HTML export,
-> KaTeX, Mermaid, and all REPL features work without it.
+> PDF export (`/studio-pdf`, `studio_export_pdf`) uses **`typst`**, shipped in
+> 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
+> fallback. HTML export, KaTeX, Mermaid, and all REPL features work regardless.
 
 ### Graphviz diagrams in Studio: `dot-watch`