Add Hugging Face Space runtime guidance

.hfignore

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+# NaturalCAD Hugging Face Space ignore guidance
+
+.git/
+archive/
+apps/backend-api/
+apps/web-visualizer/
+packages/
+examples/
+**/__pycache__/
+**/.DS_Store
+**/.venv/
+**/node_modules/
+**/dist/
+apps/gradio-demo/artifacts/runs/*
+apps/gradio-demo/artifacts/logs/*
+!apps/gradio-demo/artifacts/runs/.gitkeep
+!apps/gradio-demo/artifacts/logs/.gitkeep

docs/space-runtime-notes.md

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+# NaturalCAD Space Runtime Notes
+
+## Intention
+
+The Hugging Face Space should run the leanest useful version of NaturalCAD.
+
+## What the Space really needs
+
+- `app.py`
+- root `requirements.txt`
+- `apps/gradio-demo/app/main.py`
+- selected branding assets used by the README
+- lightweight artifacts directories with `.gitkeep`
+
+## What the Space does not need for MVP runtime
+
+- `apps/backend-api/`
+- `apps/web-visualizer/`
+- legacy archive material
+- local generated runs/log files
+
+## Why `.hfignore` exists
+
+The repository can still hold planning material and future infrastructure, while the Space runtime stays lighter and less confusing.