Nature and Floors (3D Coexistence Contract)

In the 3D demo, floors are ground overlays, not column occupants. A plant and floor can share a column in either commit order; the plant keeps the occupant slot:

• A plant does not veto a floor, and the floor does not take the plant's slot.
• A floor does not veto a plant; the plant commits over it.
• Walls can mount on a shared plant-and-floor column because the floor publishes the floor family. A plant-only column is not a wall mount.

This contract is covered by test/demos/integration/3d/floor_overlay_coexistence_test.gd. Do not add a rule that makes plants evict floors or floors veto plants unless the game intentionally changes this behavior.