Scene assembly

Each scene object has one named root. Put position, heading, rigidbody, collision, and scripts on the root, and the calibrated visual from calibrate-model below it. Apply calibration once; if the tuning record is missing, calibrate first.

• Name roots by role, with predictable names for repeated instances.
• Size colliders from measured bounds and keep them aligned with render bounds.
• Space roots by the scaled footprint in the tuning record, not by pivot position. A non-90° yaw rotates the footprint, so reserve its larger horizontal extent.
• Keep UI, effects, cameras, and lights outside model hierarchies.
• Remove the root through its surface's lifecycle and unregister events it owns.
• Rigid bodies need Ammo loaded first (build-app).

Check placement

After a rendered frame, measure each support or attachment at the object's actual position, using a mount point, raycast, or geometry sample read through inspect-runtime. A positive gap floats; a negative gap penetrates. Overlapping world AABBs between unrelated roots are a failure unless two different views prove the overlap is empty space.

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