Function instantiate_subtree 

pub fn instantiate_subtree(
    world: &mut World,
    recipe_bytes: &[u8],
) -> Result<EntityId, DeserializationError>

Inverse of serialize_subtree. Decodes the recipe bytes and spawns the subtree into world, returning the new root’s EntityId so the caller can position it / reparent it / treat it as the drop target’s child.

The root lands at world root (no SetParent is emitted for it, because the original parent edge was filtered out at serialize time).