Function base_uri 

pub fn base_uri<N: DomNavigator>(
    context: &mut DynamicContext<'_, N>,
    args: Vec<XPathValue<N>>,
) -> Result<XPathValue<N>, XPathError>

Implements fn:base-uri - returns the base URI of a node.

Walks ancestor chain for xml:base attributes and resolves against static base URI. Returns Empty if base URI is not available.