Blender render and export

Use scripts/render_or_export.py for the fragile final write. It supports one operation per invocation: render_still, export_glb, or export_usd. Read references/render-export-contract.md before choosing settings.

For MCP, prepend RENDER_EXPORT_REQUEST and append the entire helper source:

RENDER_EXPORT_REQUEST = {
    "operation": "render_still",
    "output_path": "/absolute/caller-owned/preview.png",
    "camera_name": "CAM-product",
    "frame": 1,
    "resolution": [960, 540, 100],
}

For a saved derivative: