Function atomize_node 

pub fn atomize_node<N: DomNavigator>(
    nav: &N,
) -> Result<Option<XmlValue>, XPathError>

Atomize a navigator node to its XDM atomic value.

Interprets TypedValue with proper error handling:

• Value(v) → Ok(Some(v))
• Untyped → Ok(Some(untypedAtomic(string-value))) (or xs:string for comment/PI)
• Nilled → Ok(None) (empty sequence)
• Absent → Err(FOTY0012)