docs: add engine assembly milestone concept and startup/shutdown playbook

sec: harden modal worker, update deployment docs, and add pre-push safety checks

NaturalCAD: fix Hugging Face Space metadata

NaturalCAD: add hosted backend flow and Space fixes

Switch Hugging Face Space to Docker SDK to fix build123d dependencies

Quote Hugging Face Python version

Pin Hugging Face Space Python version

Add hero image to README body

Update NaturalCAD branding assets

Initial NaturalCAD MVP