ci(plantuml): install graphviz on runner

PlantUML's -version probe exits 16 when dot is absent, failing the
step even though smetana bypasses Graphviz for our diagrams.
Installing it explicitly keeps the smoke test meaningful and removes
the failure mode for any future non-smetana diagram.

Author: jknoxdev

.github/workflows/render-diagrams.yml

@@ -30,6 +30,9 @@ jobs:
           distribution: temurin
           java-version: '17'
 
+      - name: Install Graphviz
+        run: sudo apt-get update -q && sudo apt-get install -y graphviz
+
       - name: Fetch PlantUML jar
         run: |
           curl -fsSL -o plantuml.jar \