Fail the gen workflow when the generated dialects differ

The artifact is uploaded first, so a run whose output differs still
hands over the files.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VrrewZQhCf82hp68FWNFLj

Author: kyleconroy

.github/workflows/gen.yml

@@ -33,3 +33,5 @@ jobs:
       with:
         name: dialects
         path: internal/engine/*/dialect
+    - name: Fail if the committed dialects differ
+      run: git add -N internal/engine && git diff --exit-code --stat -- internal/engine