Function write_volumetric_stack 

pub fn write_volumetric_stack<W: Write>(
    writer: &mut XmlWriter<W>,
    stack: &VolumetricStack,
) -> Result<()>

Writes a <volumetricstack> element with its child layers and volumetricrefs.

Element names are unqualified (no namespace prefix) because the volumetric namespace URI is declared on the root model element as xmlns:v.

Note: The version field of VolumetricStack is intentionally NOT emitted. The parser never reads it from XML, so writing it would break roundtrip symmetry.