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.

scripts/smoke-test.sh

@@ -6,6 +6,7 @@
 # Verifies:
 #   - pi binary present and (if EXPECTED_PI_VERSION set) matches CI's resolved version
 #   - new v1.0.0 base additions (pandoc, graphviz, imagemagick, yq, tealdeer)
+#   - typst PDF engine for pandoc (Unreleased) — `pandoc --pdf-engine=typst`
 #   - non-modal editors nano + micro (alongside nvim)
 #   - tmux 0-indexing baked in /etc/tmux.conf (required for pi-studio variants)
 #   - pi-toolkit cloned at /opt/pi-toolkit
@@ -78,6 +79,8 @@ run "micro"           "micro --version"
 run "mempalace-mcp"   "mempalace-mcp --help"
 # v1.0.0 base additions — verify presence and basic functionality.
 run "pandoc"          "pandoc --version"
+run "typst"           "typst --version"
+run "pandoc+typst PDF engine" "printf '# hi\n' | pandoc --pdf-engine=typst -o /tmp/_smoke.pdf - && test -s /tmp/_smoke.pdf; rm -f /tmp/_smoke.pdf"
 run "graphviz (dot)"  "dot -V"
 run "imagemagick"     "magick --version"
 run "yq (mikefarah v4)" "yq --version | grep -qE 'mikefarah.*version v4'"