fix: add --root flag to typst compile in docs workflow

The Makefile uses `--root ..` when compiling from docs/paper/, but the
CI workflow was missing it, causing Typst to reject the relative path
to reduction_graph.json as outside the project root.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: GiggleLiu

.github/workflows/docs.yml

@@ -42,7 +42,7 @@ jobs:
         run: mdbook build
 
       - name: Build examples and PDF
-        run: make examples && typst compile docs/paper/reductions.typ book/reductions.pdf
+        run: make examples && typst compile --root docs docs/paper/reductions.typ book/reductions.pdf
 
       - name: Build rustdoc
         run: RUSTDOCFLAGS="--default-theme=dark" cargo doc --all-features --no-deps