feat: distributed rendering with Celery

- celery_app.py: Celery configuration with Redis broker
- tasks.py: render_effect task with full provenance tracking
- render.py: CLI for submitting render jobs
- Successfully renders cat → dog with provenance chain

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

requirements.txt

@@ -0,0 +1,5 @@
+celery[redis]>=5.3.0
+redis>=5.0.0
+requests>=2.31.0
+# Core artdag from GitHub
+git+https://github.com/gilesbradshaw/art-dag.git